Believe it or not, if you are running Windows XP with Service Pack 2 or later (most people are); then you put the virus on the computer yourself. (or someone in your home). You cannot leave the computer unattended and have a virus crawl in while you're not looking (it simply won't happen) and you cannot get any sort of virus / spyware from merely going to a website. (no matter how bad the website is). Believe it or not, at some point, someone on your computer did choose to install this virus. They may have thought they were installing an online video game, the latest audio / video plugin or it may have just been an ActiveX installation that prompted you to allow or disallow an online ActiveX install. Regardless, you or someone using your computer actively chose to install this program that ended up being a virus. This is why as a general rule of thumb, if you don't absolutely, positively need to install something, then don't.
One of the most common places virus distribute is through free online computer security and repair tools. The virus distributor knows that your computer is unprotected (why else would you be downloading free antivirus). There's no such thing as a free lunch and the few free antivirus out there that are legitimate aren't that good anyway; after all these are products that have zero funding and zero dollars to pay staff to update and improve the program, how good could they really be. The cheapest antivirus is truly the one that works well and prevents the need for computer repairs.