If you already have a keylogger and/or a program taking snapshots, then you should already have proof as to whether someone is using your PC. So if you still suspect it but have no proof yet, then either there isn't anyone using it, or they are turning off the spyware and/or deleting any logged info.
One sure bet however, is to start the system with a command prompt only, and check the date and time of the registry or other such files, as a number of them get updated every time the system is used.
Also, unless the user is aware of it, the recent documents list can leave tracks too.
