There probably is. But you could also figure it out by looking at the Event Viewer in the Admin Tools control panel.
the Eventvwr.msc has all the logon/logoff info in the Security Log but you need Admin Privs to even view that data. The Event Log Explorer can be installed and set to always use the Admin account and make access much easier.
You find the Logon event and then the corresponding Logoff event.
You do the math to get user session time.
I would assume you to put the data into a spread sheet and get Excel to do that for you.
Monitoring PC use.
There are a couple of free programs I know of, that might do what you want.
you can user PC ACME
Thats nifty, may have to get that just for fun.
Boztech VENM if you are willing to pay the price. This software can do anything remotely (almost)
AIDA64 will track boot times, also there is a remote capabilities with one of their versions... again if you are willing to pay the price.
Otherwise, just open up the Task Manager and click on processes and then View at the top, then Select Columns... and check CPU Time. This will give you how long the computer has been up if you look at the right process... this may not work in Windows 7 or Vista. So good for XP, I do know that.
Why don't you just use Task Manager?
I came across this User activity monitoring for PC