• 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.

• 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.
  
• Internet Explorer 6, from the list of old Internet Explorer downloads at OldApps
• Windows 98 service pack 2, from the list of Windows 98 SE service packs at MGDx
• DirectX 9.0c, from the repository of old versions of DirextX for Windows 98 at OldApps
• vbrun300.dll, from Microsoft's website
  
• 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
  
• Java 5.0 SE Runtime Evnvironment
• Misc. tools
• Adobe Reader 6.0, from the Adobe FTP site (Oldest Windows O/S supported by Acrobat 10.0 is Windows 2000)