I’m not sure how extensive of a program you want but based on your question there are several programs out there to help you. As for the rest of you who answered her questions: If you can’t or don’t want to answer the question then you shouldn’t answer it at all. She was answered with questions rather than answers. She doesn’t want to change her password or use the, almost useless, event viewer. She wants to catch someone who shouldn’t be using her computer to begin with! And putting powder on her keyboard is an answer? Yeah right! But instead of using powder use super glue, that way the culprit will be there when you get home from work. Sheesh! One of the few people to give you good advice (when I posted my comment) and actually read what you had posted was accuratellytrue who suggested that you use a keyboard logger. This is exactly what you need. Many keyboard loggers are expensive but there are many good free ones also.
These programs will tell you when a computer is started and when it is shut off. In addition (some of them) will tell you what applications are being used, what things are being typed and some of them will even take a snapshot of the computer screen seen as the person who is using your computer views it. I will show you some free software and their web sites.
Simple Activity Logger is small easy to use but limited in its features. It is freeware, which means it’s free. Here is the web address: http://www.exefind.com/simpleactivitylogger-P22096.html
User Logger is another more powerful program that even takes snapshots during the day at intervals that you decide. Here is the web address: http://www.snapfiles.com/reviews/User_Logger/usrlogger.html
Tiny Key Logger is a very small invisible program that logs everything. Here is the web address: http://www.snapfiles.com/reviews/Tiny_Key_Logger/tinylogger.html
I have not tried these programs but from the sound of it I like Tiny Key Logger. The only caveat being that this is not for the average person because you have to tell the computer what programs to look for when someone is using the computer in the more advanced settings.
I like User logger the best although I haven’t tried it. From what I’ve read I think that the program best suites your needs and will give you all the information you need.
If you want to look for other programs you can use a search engine and search for
Keyboard logger
And
Freeware keyboard logger