W32.Welchia.worm is thenew 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 will probably be needed after you reset the date. Will update when we find out more.
Eric