This has happened to me on many occasions. Even 3rd party software wouldn't allow any adding/removing.
Another mysterious symptom has been Ctrl/Alt/Del would add 25% to my cpu usage EACH TIME it was used (ctrl/alt/del 4 times = 100% cpu usage).
Each time this has happened to me, it's been a virus that I spent days trying to find. I'd even run anti-rootkit software.
I never really had much luck figuring out how to solve it, other than reinstalling the OS over itself.