W32.Welchia.worm is the new one. It is actually written to fix the Blaster worm from last week. It doesn't harm your system but does cause network degradation from traffic.
To fix your system, set your date to the year 2004 and it is supposed to delete and remove itself.
This should take care of it.
[edit] It doesn't say but a reboot is probably neede after reseting the date. Will update as we find new info.
Eric