If you are going to fix a problem like this you must first make sure you are not connected to the internet. Pre-download "ad-aware" and "avg free" virus checker from
www.download.com and install and run them.
This may take ages, it would be quicker (but more complicated) to reinstall the OS.
Once done, switch on Windows Firewell before you go back on the internet (or download zonealarm from
www.download.com and install it instead).