Boot CD

When your computer is in a state where it can't boot, you may be able to fix it with a bootable CD or USB memory stick.

The Universal Boot CD for Windows (UBCD4WIN) can create a boot CD or USB stick but requires Windows XP installation files. It is available at http://www.ubcd4win.com/.

Hiren's Boot CD is a complete CD image. You just need to burn it to a CD. Search for it on Wikipedia.

Both of these will start a basic Windows XP desktop which includes most system tools and some form of anti-virus program.