In addition to my previous post (which was more geared on how to avoid the issue), here is more info about what to do if you actually are currently impacted, and constantly rebooting.
1. Upon boot-up, press F8 immediately after the BIOS initializes, and select Windows Safe Mode with Networking by using the arrow keys.
2. If prompted, choose the Windows XP Professional menu option.
3. When the login screen appears, log in as appropriate. There might be a longer than usual delay for the login to complete.
4. In the VirusScan Console, double-click on the Quarantine Manager Policy.
5. In the Quarantine Manager Policy window, select the Manager tab.
6. If there is any instance of W32/Wecorl.a virus listed, select one and right click to select Restore. This should restore the file (C:\Windows\System32\svchost.exe) back to its original location. It might prompt you to confirm your selection.
7. Repeat steps 1-3 to re-enter Windows Safe Mode with Networking
8. Download and run the fixed 5959 dat update here:
http://www.mcafee.com/apps/downloads/security_updates/superdat.asp?region=us&segment=enterprise
If at any point you get a notice that there was an error and your PC will reboot in xx seconds, you can stop that by going to Start, Run and enter "shutdown -a" (without the quotes) to abort the shutdown, so you can continue working on restoring the svchost.exe file.
Good luck.
(P.S. - I Don't work for McAfee, and am glad because I expect their stock to take a hit for this outrage.)