Following are various guides I've created for tweaking and adjusting several popular operating systems.

Windows XP	A detailed guide on tweaking Windows XP for maximum performance.   Introduction   GUI (graphical user interface) tweaks   FS (file system) tweaks   Memory tweaks   Windows Services   Network tweaks   Misc Tweaks
Windows 2000	A guide on optimizing Windows 2000 workstation for optimal performance.   Introduction   Windows 2000 Network Tweaks   Windows 2000 Speed Tweaks
Windows 9x	These are tweaks for Windows 9x (Windows 95, 98, 98se and ME).   Introduction   Windows 9x Registry Tweaks   Windows 9x Speed Tweaks