A good virus will watch the registry since it is typically executed from 2 or more locations. Try and delete one, and it will automatically replace it.
Same with BHO's. Delete one, and a good virus or Adware program will automatically replace it.
The only real way to delete one is to use software, or to do it manually from a safe mode or even a command prompt repair mode from your XP CD.
Spybot however will get rid of what it is you have issue with. Spybot however cannot repair any damage caused by whatever you have so if it has physically damaged something, not much you can do about it.
Skybird mentions he doesn't like repair mode since he doesn't trust it. That is nonsense. Repair mode however is a last resort since you will be loading patches again to fix holes.
-S
__________________
 
|