Sounds like you'd need a program to do that. You may be able to download one, but why not write it yourself? The hardest part is the calculations.
Output the time and date to a file, then upon execution of the program have it read those two values (place a simple file in either the startup folder or autoexec.bat) and calculate how much time the computer has been on, or at least loaded that part of the OS... Simple, effective, uses only a little hard drive space (less than 1k, I'm sure), and only uses RAM during program execution.