Running Windows? - remember to use Windows Update
The gaps in between major worm break-outs appear to be decreasing, the latest one that has hit in the last week has been the Sasser worm. This worm does not spread by email, but by simply exploiting machines it finds over the Internet that this have a vulnerability in Windows exposed via port 445.
Microsoft have issued a security bulletin, which is available here. The quick summary is download the correct update for your OS, before you become part of the problem. People running a PC behind a well configured software/hardware firewall or even a NAT router are most likely to be OK, but installing the patch is still advisable.