You will probably need to do multiple anti-virus/malware scans with different programs, since not every program detects every single virus.  I'm guessing that some programs can also deal with ones that other programs cannot. The good news is that there are enough good free anti-virus software programs that you can easily download them.I use AVG anti-virus (the one ski mentioned) and have it set to scan automatically once or twice every week.  You can turn off certain functions of AVG (e.g. the internet link scanner) so that it doesn't seem to take over your system if you want.  My computer doesn't run slow, but I suppose it could depending on the way your computer is set up.Here are other ones I use:Malwarebyteshttp://www.malwarebytes.org/SuperAntiSpywarehttp://www.superantispyware.com/download.htmla-squared Free - make sure to try this onehttp://www.emsisoft.com/en/software/download/(go down to "a-squared Free 4.5")I would download at least two of them and try them out to see if they can fix the problem.  It might take 1-2 hours to complete a scan, depending on the number of files on your computer.  You can configure these programs so that they don't run when you start up windows, but instead only manually (that's what I do).  Then in the future, you'll probably want to run them on occasion in the future.If these don't work, there are other ways to address your problem that I can let you know about, like using a rootkit scanner.