If you are running Windows XP, NT, or 2000, you are vulnerable to a major security hole. Right, no surprises there, but this one allows a remote attacker to compromise an out-of-the-box Windows install.

http://support.microsoft.com/?kbid=823980

Visit that link and scroll down to find patch download links for your particular operating system. This security hole will allow non-firewalled default Windows XP/2K/NT installations to be cracked remotely. There is already a worm on the loose which is automatically exploiting this hole, and then using the newly infected computer to propagate itself, as worms are wont to do.

Act now-ish. Worms like this spread ridiculously quickly.

edit: For ease of access, here are convenient download links:
Windows XP Home and Pro 64-bit
Windows 2000 All versions
And just for the sake of being complete, even though people who are running this should have patched a month ago:
Windows NT 4.0 Server Terminal Server
Windows 2003 Server 32-bit 64-bit

Do I need 64-bit XP? If you have to ask, no.