You've got 20 minutes to get your system patched:
"If the human body did patch management the way (companies do), we'd all be dead." - Fred Baumhardt. Microsoft Security Consultant
According to the SANS Internet Storm Center, an unprotected PC connected to the net to get patches, updates, and security fixes will last about 20 minutes, on average, before becoming compromised. Last year, the number was 40 minutes.
SANS offers a helpful guide called Windows XP - Surviving the First Day. ZDNet has a brief overview of this recent analysis."