WEll other then reseting the cmos... she will know that you were in then.. what you could do is spend like $50 buy a keyboard logger.. it plugs in between the keyboard and the computer and logs everything. Depends on the size some are like 15k , and seen as high as 126k doesnt sound very big but then again its just text. that would be the most stealthy way to go at doing that. most people don't look at the back of thier puters before using them.
Jerms