This essential if you like upgrading your PC regulary - Belarc System Advisor. Simply scans your system and tells you exactly what's on it and in detail. Processor, motherboard, graphix card, RAM, whats on your hard-drive, system protection, everything. Great for making sure you buy compatible RAM.
http://www.belarc.com/free_download.html
Totally free of course.
I would also like to join Arab_Freak up there in procaliming the wonder of Crap Cleaner. The best win-cleaner programme ever. AND FREE. (I lost over 800MB - my Pc is running so much smoother!)