1. What OS are you running, and did you install it by using the default method.
2. Did you use a logon password before getting hacked.
The reason i asked is it could be as simple as a remote desktop, or file share intrution. Im not saying everyone else is wrong, or that you should not format and reinstall. But consider this, if you do reinstall and fail to secure your system you may end up wit the same problem.

I take it Ill get a "you will be banned warning" for pointing this out, too.