Read about the problem at Slashdot.maxyimus
I had the same problem. Since I didn't have time to identify which one of today's updates caused the problem, I removed them all and now my computer is back to normal.
Follow these steps:
1. Boot from your Windows XP CD or DVD and start the recovery console (see this Microsoft article for help with this step)
Once you are in the Repair Screen..
2. Type this command: CHDIR $NtUninstallKB978262 $\spuninst
3. Type this command: BATCH spuninst.txt
4. Type this command: systemroot
5. Repeat steps 2 - 4 for each of the following updates provided by FindMeFollowMe:
6. When complete, type this command: exit
Your computer should restart and everything should be back to normal.