Boot into Safe Mode by clicking F8 repeatedly while booting up and then locate System Restore (Start/All Programs/Accessories/System Tools) and use that to go back to before this happened.
Don't download software that you are not sure about. No software is needed to activate any kind of Windows whether it be XP, Vista or Windows 7, ... that function is built-in.
If the restore function is successful then turn off System Restore to delete that bad restore point.
Help with that here: How to enable or disable System Restore in Vista
Re-enable it after you are sure that everything is OK.
I suggest strongly that you back up all your important documents and folders to either an external hard drive or external media such as re-recordable CD's (CDRW media) or DVD's (DVDRW's). Then if this kind of thing occurs you wont lose anything if you have to format and reinstall your system.