The Windows Disk Cleanup utility does a good job of deleting old Microsoft temp files, logs, memory dumps and other OS related junk files. It can free up a lot of space on your computer but it often leaves a lot of junk behind. A great program that I use all the time on my computers is called CCleaner. Not only does it clean the Microsoft OS related junk files from your computer, it also removes junk files from other popular applications such as Firefox. Additionally, it is very good at locating other temporary files.
Visit CCleaner.com to download a free copy.