If your windows system doesn't start (e.g. due to an virus infection, like the new Facebook virus) it could help to re-write the Bootsector / Master boot record.
This is (in my opinion) the last thing to try before you format your HDD.
Be aware, it could bring up some problems when more Operating Systems are installed (Like a Linux Bootloader etc) .
http://it-tutorials4you.blogspot.com/...
A new Facebook virus is out there, it overwrites your MBR, so you can't boot into windows. This should fix it.
mia050 compilation - Sense - Likemine
Link:
http://www.archive.org/details/mia050