'An unprotected computer running Windows XP survives an average of 26 minutes on the Internet before hackers identify it as vulnerable.'
--The SANS Institute, a cooperative Internet security organization
WOW! It's even worse than I thought! You can read the entire article here.
If you haven't yet invested in anti-virus software I thought you might be interested to know that there are some excellent free products that may be downloaded. These are intended for HOME use only, and ARE NOT to be distributed to an entire school or business.
Avast 4 Home
If you think your computer is already infected, try McAfee's AVERT Stinger.
And remember, use just one anti-virus software product and keep it updated to provide protection against the latest mutations and variations that occur almost daily.