Do the following:
1. Disable System Restore.
How to Disable System Restore on Windows ME, Windows XP and Windows Vista:
http://www.pchell.com/virus/systemrestore.shtml
2. Boot into Safe Mode.
How to start Windows in Safe Mode:
http://www.pchell.com/support/safemode.shtml
3. Scan with Avira.
http://www.free-av.com/en/download/1/avira_antivir_personal__free_antivirus.html
4. Scan with Malwarebytes.
http://www.malwarebytes.org/
5. Reboot back to normal mode and re-enable System Restore.
Note: Safe Mode is needed in order to prevent the malware from running in memory, which may make it hard to remove it.
In the future, always scan any file you download before opening them. That 'keygen' could have been a lot more malicious and even wiped out your hard drive!