Have you run your anitvirus software regularly? We use Norton. We have some nasty parasite (not virus) take over one of our computers a couple of weeks back and called in the local Computer Doc. She ended up reformatting the hard drive (but did manage to save nearly all our data files). She recommended Spy Sweeper by Webroot - you can download a free version but if you want updates you need to subscribe for about $30 a year. We've also used Spybot Search & Destroy, basic version free online, and Ad-Aware which alos offers a free version. I run them once a day (twice if my kids have been online a lot) and they're good at finding and removing trojans, spyware, unauthorized programs that have snuck onto your system, etc. For a quick check of what parasites may be on your computer, go to
http://www.doxdesk.com/parasite/
There's lots of information here, on the bugs and removal methods. Hope this helps!