Craig Newmark linked to this PC Mag article last week. I read it and installed a couple of the suggested programs to help me clear my computer of various nasties. If you are having problems with your computer, give it a read and make sure to click on the links which will take you to more indepth reviews of the products discussed. For example:
Don't assume that your friend with the problem has been visiting shady Internet sites. Perfectly clean sites have been hacked in such a way that simply opening the page downloads malicious software to the victim's system. Worse, many modern threats include countermeasures to fight back when you try to install antivirus software. Some tweak the Registry to disable launching of known antivirus executables. Others monitor Internet access and specifically prevent access to security company websites, so you can't update or register the product. They're ingenious, in a bad way.
When you can't install the full power of an antivirus solution, you may be able to wipe the system clean with a free cleanup-only tool. Malwarebytes' Anti-Malware Free may be the best known tool in this category. Tech support agents in some security companies routinely use it to help users who run into trouble at installation. A few years ago I even encountered a Symantec tech support agent using it (strictly against Symantec policy).
The link to Malwarebyte takes you to a more indpeth review of that particular piece of software. I downloaded the Malware piece and the AVG Anti-virus Free 2013 and those seem to be decent programs. Since I am not a experienced computer tech guy, I did not download Norton Power Eraser because using it may direct me to delete programs that my computer will not work without.