User the recovery disks that came with the system, or from the restore partition of the computer. If you don't have either available, you can contact the PC vendor for a disk set, but will probably have to pay for that.
I suspect the WGA tool found the pirate version...and just wait until MS change the tool again to find all those Ultimates lollll thats going to be fun
Combine the the human qualities of cheap, stupid, and the larceny of wanting something for nothing (variation is MS is big and rich I am entitled to steal from them) and you get a reason. Ssjensheer I suggest you purchase an OEM version and clean install it on a formatted drive. Of all the software a system uses you would think people would want their OS to be secure. BTW it is not so hard to embed malware ito a pirated version of the OS. GL to you.