Zero-cost, three-step procedure if your PC might have become infected as the result of installing a program you downloaded.

(a) First, upload the installation file of the program you installed to Jotti.org [1] for a free scan. Jotti will then run it through more than a dozen malware scanners and let you know if there is a problem.

If Jotti determines that your file is clean, it doesn't mean that there is no infection. It simply means that it's unlikely there is an infection, and that folks, is a very comforting finding.

(b) Download and run the free Panda Rootkit detector [2]. Again, a clean scan is not a 100% guarantee of no infection, but should add greatly to your confidence. Panda doesn't run on Vista, so Vista users should use the BlackLight [3] anti-rootkit scanner instead.

(c) Finally, download HijackThis from this page [4], and follow the instructions on the same page which tell you how to create a log that you can paste to web forums.