In the past I have recommended the anti virus software Ad-Aware - but I have now withdrawn that recommendation.
I have had reports from readers that the latest version (version 10) seems to be messing up some computers, and this is confirmed in online forums (click here and scroll down). It is best avoided, at least for the time being.
They have also made it more difficult to get the free version, which is irritating, so I advise Oldie readers to stay clear until the news is better. I'll keep an eye on it.
You can read about the software I do recommend by clicking here.
The reports online of problems seem to have died down, so the issue may have been resolved - however I have not tried it myself.