We have seen a massive increase in "fake virus" warnings infecting computer systems, but they are not viruses, so most antivirus software will ignores them.

They may ask you to pay for your computer to be cleaned, do not do it, they could then get your credit card details.

They can infect your computer in a number of ways, you will have visited an infected website or clicked on link in facebook or myspace which looked to be from a friend.  This is more likly to happen if their computer has been compromised.  The link may be a message indicating that there is a picture of you on the internet and click here to see it, you are then redirected to a website which says to view this image you need to download an update to a viewer program, this is the scamware.

If someone sends you a link, always check with them to make sure they have "actually" sent the link before clicking on it.