Didn't you read http://support.microsoft.com/kb/223316 ?It tells how to make a backup copy of the key. It would have saved you quite a lot of money.
And did you know that - even if Windows doesn't run anymore - it's free to boot from a Ubuntu disk and copy your files to a USB-stick or external drive?
Depending on what you mean with "can't access" it could be that you should take ownership moreover. That is if you happen to get an "access denied" error. Google XP TAKE OWNERSHIP to learn how to do that.