ccleaner.com - cleans unnecessary files from your C drive.
Startup Inspector for Windows - lets you stop unnecessary files from funning in th background when windows starts.
FreeRAM XP Pro - diisplays in real-time how much RAM your PC is using, and can free up space when RAM drops below a specified level.
All available as freeware from download.com.
But be careful with what you allow ccleaner to delete, and with what you allow Startup Inspector to de-activate at Windows Start-up.