I echo Chris's comments.

Two other things to lower the likelihood of invasion by unwelcome virus, and malware attacks.

> Do not use Internet Explorer. Go to Mozilla Firefox or Google Chrome. 

> Do not use Outlook or Outlook Express. Go with Mozilla Thurnderbird. A browser based email account will also keep you "cleaner" than a local machine application.

Many times a virus is "constructed" or written to target computers that use Microsoft browser or email, as those are the norm, and honestly easier to find security breaches in them.

Before I went to Mozilla products, I was dedicated Netscape user, since 1996. While I have had a few problems, it's nothing like a lot of the horror stories I hear.