It's a given that operating systems and large software suites have bugs, and Windows is definitely a member of both categories. The Microsoft method of dealing with bugs in Windows has varied depending on the product line in question.

In the consumer Windows 9x product line, users were largely left on their own. Only the most severe security bugs were addressed, and even then, for the most part, computer owners had to take the initiative to visit Windows Update to download "Critical Updates." It took many yearsand tens of millions of virus-infected computersfor Microsoft to develop an automated method of delivering security updates to Windows 9x users, and even then they had to download this updating mechanism themselves. Bugs that affected Windows functionality but not security were largely left unfixed, with the result that a typical Windows 9x could be expected to crash once a day, in heavy use. Patches called hotfixes were available as downloads from support.microsoft.com for some problems, but finding these was a job that required a high level of training and technological savvy.

However, Microsoft recognized from the start that operating system products sold to business customers had to have a level of support that rivaled that of other enterprise software vendors, so the Windows NT product line has enjoyed a much higher level of support. This is not a trivial or inexpensive undertaking, as proper software quality management is expensive, time-consuming, and involves interactions with customers on several levels:

• Communicating known bugs to customers.
• Development and announcement of any possible short-term workarounds, and estimated delivery dates for complete fixes.
• Thorough testing of all software changes for possible negative interactions with existing hardware, software, and all previous updates (regression testing). If done properly, this involves testing installation and operation of all fixes on every possible combination of supported hardware and software, with and without every combination of other optional software components, versions, patches, updates, and fixes. Testing a single software change against all of these permutations takes an enormous effort.
• Timely release of individual fixes to customers who are encountering a given problem.
• Periodic bulk updates that combine all known and tested fixes, to simplify the maintenance job for the majority of customers who are not directly affected by each identified problem. This can be accomplished by releasing new operating system versions, or through an updating mechanism.

To meet these needs, Microsoft has established a system for delivering updates for the Windows NT product line that includes Windows 2000 and XP, with five mechanisms:

• The Microsoft Knowledge Base contains reports on known problems and solutions, at support.microsoft.com.
• Hotfixes, interim patches that address known problems, which are made available for download at support.microsoft.com. Hotfixes are intended for use only by customers directly experiencing a given problem (due to, for example, a particular combination of hardware and software that exposes a given bug). They are not necessarily tested with every possible combination of hardware and software. Several hotfixes are typically posted every week.
• Service Packs are cumulative updates of all hotfixes released since the operating system was original released. When Microsoft constructs a Service Pack, hotfixes are tested much more extensively. In addition, new functionality might be included, as we saw with the inclusion of Windows Firewall in Windows XP Service Pack 2, although Microsoft has stated that they expect this to be a rare occurrence.

Service Packs are generally tested internally by Microsoft, and then by major computer vendors, and then by large corporate clients and possibly the public, through a beta program, and may go through two or more beta versions before being finalized. Service Packs have typically been released six months to two years apart.

• Rollups, which contain a significant set of hotfixes released since the last service pack. Rollups include only the most important hotfixes, ones likely to affect a large number of customers, and are tested for proper installation and correct operation together as a package. Testing is less extensive than for a service pack. Rollups are designed to make installation of important hotfixes easier, and to make them available in a timely fashion without the burden and expense of a full-scale Service Pack. Rollups are produced only on rare occasion.
• Critical Updates are patches that address a security risk to the average user. Critical Updates are typically given extensive testing, as they're considered mandatory for all customers. They are distributed through Windows Update, and now through the new Automatic Updates system so that all users will get them in a timely fashion. Critical Updates are typically released once a month on a Tuesday, unless they are considered urgent enough to be released immediately after testing.
• Optional Updates are updates to Windows components, new versions of Windows applications, or new functions that have no direct security or usability impact, but which for some reason Microsoft feels it is important to distribute. They're made available through the Windows Update tool.

Now that Windows XP Home Edition has brought NT to the consumer Windows product line, this level of support is available to the average consumer as well.

It's a matter of speculation what will happen to the frequency of issuance of Service Packs for Windows XP, as its increased reliability coupled with a regular stream of bug fixes may prove to be a disincentive for purchasing upgrades to later operating systems.

Service packs can be obtained in several ways:

• On CDs and DVDs, downloaded from Microsoft. These versions contain all of the files needed by any installation of Windows, and typically covers all flavors of the operating system. For example, updates for Windows XP Home Edition, Professional, Media Center Edition, and Tablet PC Edition, are all in the CD version of a Windows XP service pack. At the time this was written, it was not certain that a service pack for Windows XP 64-bit versions would be packaged on the same CD as the 32-bit version, but it is likely that it would be.
• Via download from Windows Update. The update process first downloads a small tool that analyzes the user's Windows installation and determines which updated files are required. Just the files needed by the user's flavor for Windows are retrieved and installed. This is the easiest download format for users with just one or two computers.
• Via download from support.microsoft.com. For sites that must update several Windows computers, the entire Service Pack distribution can be downloaded and stored on a shared network folder. This format is preferred by sites that have many computers to update.

In addition, service packs can be merged into the Windows installation setup files in an operation called slipstreaming. Organizations and vendors that install many copies of Windows can use this technique to construct updated Windows installation media. (In addition, a Windows Deployment Toolkit can be obtained from microsoft.com, which lets organizations pre-install applications and customizations onto new computers.)