Ring up windows and tell them what happened, they usually dont believe you or you will probably get Ahmed from Tanzania and he won't understand what they hell you're saying, but keep trying, if they don't get you start again and eventually you will get through to someone.
This has happened loads to me. 3 options if they dont believe you, as they probably think you have a fake copy:
1)Use it for 29 days and before the activation period is up reformat your pc again saving the files you need to another drive or on disk, and then you can use it for another 29 days again
2) get a new windows - lots of money
3) get someone who knows about computers to sort it out. My uncle always sorts mine out through DIS somehow, but i also have about 5 windows disks, cos i keep having to do this when my pc gets so screwed up it wont work without reformatting.
In any case, ring them up first and try your luck