CPU utilization

[user at domain.invalid]

I am writing a diagnostics program and would like to get the CPU 
utilization for the system. Does anyone know simple way to do this?
preferably platform independent for future porting, but for now I need 
to get it to work on WIN32. Also if there is a way to get averages (1 
min avg, five min avg, etc.) that would be great.
Any help on this will be greatly apprieciated.
Thanks,
Eric Jensen