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.