This has to be in the running for one of the STUPIDIST antivirus false alarms.
TacOnlyOne is the most absolutely trivial harmless thing.
it's a registry entry used so that a program knows it should only run
one copy of itself at a time, thus the name.
any program thats flags things like this as a virus signature is engaging in the mostly lazy, shoddy, stupid, ignorant, irresponsible behavior.
any security program that blindly tags registry keys like this that having nothing to do with anything should be avoided and shunned.