Rootkits are the most nefarious of all malware, giving hackers access to your entire PC without your knowledge. They use special techniques to hide themselves from many antivirus and anti-malware programs, which makes detecting and killing them exceedingly difficult. Because of that, just using antivirus software isn't enough. Instead, you need a specialized rootkit detector and killer.
That's exactly what F-Secure's BlackLight Rootkit Eliminator does. It scans your PC for hidden processes, folders and files, then reports on what it finds. If your PC is clean, it will tell you so. If it finds anything hidden, it tells you that as well and lets you clean it up. Double-click any entry, and you'll get more information about it, such as the file location, a description and company information.
To kill a rootkit you've discovered, you have a choice of renaming or deleting the file using BlackLight's built-in tools. It's a good idea to first rename suspicious files, which gives them a .ren extension and prevents them from executing. Next, do a Google search for the file names to see whether they really are malware. Rootkits often hide legitimate files and processes, such as Explorer.exe, so make sure not to get rid of any legitimate ones. If you confirm that files are malware, then delete them.
Warning: Only very experienced users should attempt to clean their PC with this software, because if you rename or delete valid files, you can cause serious problems. If you're at all unsure about what you're doing, you might want to try a different free anti-rootkit tool called RootAlyzer, from the same folks who bring you Spybot Search & Destroy. It checks your PC for rootkits but doesn't offer tools for deleting them. (Note that RootAlyzer is still in the preview stage.)
The answer? A great Firefox extension called NoScript, which not only blocks scripts, plug-ins and various types of code, but also protects against cross-site scripting attacks. It lets you block scripts, plug-ins, and code on a site-by-site basis. You can control it to an exceptional degree, including whether to block scripts on sites on a one-time basis or permanently.