The newest threat to your sanity on the web is the fake virus detection or fake software upgrade message. These messages usually pop up while you are web surfing, but not always. What usually happens is this window pops up and reports you have viruses on your computer - usually some large number of infections. In fear of having some nasty bug on your computer you accept the offer to download and run a fix. Congratulations, you just downloaded malware or viruses onto your machine in an effort to protect yourself!
Never, and I mean NEVER, rush to download and install software on your computer when you get one of these pop-ups. What should you do? I'd recommend shutting down your computer right then and there - after saving any open documents and getting out of your other programs (just ignore the message begging you to help you fix your problems).
Reboot your computer and then run a virus scan using your existing anti-virus software. You do have anti-virus software installed already don't you? If you don't then you may want to download an antivirus program or have ACS setup your security and anti-virus systems. Here is a list of anti-virus utilities, including those that provide real-time protection.
Make sure your anti-virus software is current and then tell it to scan. If you come up clean then it was probably a fake message and you should get back to whatever you were doing before the scary pop-up window appeared!