General
Service packs are availble from Microsoft's website in their Service Pack Center. I recently wiped and reinstalled the operating system for a Windows XP machine, and found that I had to download and install service pack 2 before I could use the Windows Update link to update the O/S.
Windows XP
Linux tools
Windows Services for UNIX provides tools for connecting a Windows XP PC to a Linux/UNIX system
Windows 2000
O/S updates
Internet Explorer 6 service pack 1 (from the Microsoft Download Center) needs to be installed before you can run Windows Update. (IE6SP1 can be downloaded using another browser like FireFox, but Windows Update will only work with Internet Explorer 6 service pack 1 or later.)
Windows 2000 Service Pack 4 is a requirement, and is available as an Express Install for End Users (from the Microsoft Download Center) and as a Network Install for IT Professionals (also from the Microsoft Download Center).
While Windows 2000 has command line autocompletion (while at the command line, type a few letters, hit the [TAB] key, and the first matching file will be added to the command), the default character is not the [TAB] key. To change the autocompletion key, follow these instructions (from the Windows 2000 tips section of the ActiveWin website):
Run 'regedit'.
Find the key HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Command Processor.
Edit the DWORD key value 'CompletionChar' from the default of 0x40 to 0x09.
Web tools
While the latest version of Adobe Flash will not work with Windows 2000, Adobe Flash 10 will (from Adobe's archive of Flash player versions)
Same deal with the latest version of Adobe Shockwave; Shockwave version 9 will do the job (from the Adobe page for stand-alone installers for previous versions of Shockwave).
Windows 98/ME
O/S updates: Microsoft seems to have dropped many Windows 98 related support files, so alternate sources need to be used.
Drivers
A list of legacy drivers can be found at the public FTP site hosted by I-Shou University in Tawan
Web tools
Firefox 2.0.0.9 (Firefox 3.0 and beyond require Windows 2000 Professional or later)
Adobe Flash 9, release 280 from Adobe's legacy Flash player repository
Shockwave players 7, 8, and 9 in Adobe's repository of stand-alone installers for Shockwave player
Misc. tools
Adobe Reader 6.0, from the Adobe FTP site (Oldest Windows O/S supported by Acrobat 10.0 is Windows 2000)