Monitor Computer Usage Time

I am looking for a program which can effectively monitor the time a user spends on a computer, perhaps by looking at keystrokes/mouse movements, or something else...either way, I'd like to be able to compare the usage time on several computers. If I could monitor these times remotely, that would be great, but it is not necessary. It would also be great if rather than just saying "user x spend y time on the computer today", it would log it, so say between what times the user was active.

Does anyone know an effective program to do this?

Thanks,
Tom