To sum up everything for the inevitable entry of people who are too lazy to click links and Google (and, therefore, who will keep asking questions):
shuts your computer off just for the hell of it
To clarify... it doesn't actually shut your computer off. The MSBlast worm comes in through port 135 using a previously known vulnerability (http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/MS03-026.asp) in RPC. It attempts to determine what system you are running and then tries to exploit RPC. It often results in RPC crashing which is causing the shutdown. By default, Windoze attempts to reboot the system if RPC crashes (thus bringing RPC back to life). MSBlast, however, installs itself in the system32 directory as an autostarter and then crashes RPC again, resulting in another reboot, ad nauseum. You can stop the crashing by changing the "Action to Take" for RPC to "Take No Action" for all events (crash, etc.) but you'll still have the worm.
Once installed, the binary opens up port 4444 on your system and scans random IP addresses at port 135 looking for more vulnerable machines to propogate to, though it seems to stay in your IP block most of the time (i.e. within your ISPs block of IPs for most home users).
The bigger deal with this worm is that on August 15th at Midnight (or, the 16th, depending on how you look at it), it's going to start attacking the windowsupdate.com site in an attempt to SYN flood it (DDOS). Expect to see that start happening within 36-48 hours.
It's relatively simple to stop. In fact, in theory, if you were smart enough to turn on ICF when you setup your Inernet connection in XP, you should be safe. ICF blocks incoming port 135 requests. If you don't have ICF, you just need to run a firewall that blocks ports 135, 139, and 445 (and any others you may have configured as RCP ports for whatever reason). Also, patch your damn computer. Home users have no excuses. There are reports that the patch is ineffective, but it's better than not trying it at all.
Finally, to stop the shutdowns, simply go into the command prompt (Start > Run > cmd or, on Win9x Start > Run > dosprmpt) and type 'shutdown /a'. There are also reports that you can set the system time back an hour to delay the shutdown an hour, but I can't say for sure that that works. Go into the system32 directory and delete 'msblast.exe' and delete the registry key 'HKLM\Software\Microsoft\Windows\Run\windows auto update'.
Any more info that I forgot or corrections welcome!