All the programs below are free, small, easy to use and very powerful. They'll help you make the spring cleaning for your PC.
PageDefrag (Sysinternals) - defragments registry, event log files, Windows 2000/XP hibernation files.
NT Registry Optimizer - defragments registry files, by recreating each registry hive "from scratch", removing any slack space that may be left from previously modified or deleted keys.
Power Defragmenter GUI - GUI application for Contig (Sysinternals). It's perfect for quickly optimizing files that are continuously becoming fragmented, or that you want to ensure are in as few fragments as possible.
CrapCleaner - cleans Temporary files, URL history, cookies, autocomplete form history from IE, Firefox, Opera. Also cleans registry, temporary files from Windows and other third-party applications (eMule, Kazaa, Google Toolbar, Netscape, MS Office, Nero, etc).
FreeRAM XP Pro - read more about the freeware in this article.
Also read interesting tips about pagefile optimization.