Concerning Windows API - go to
MSDN Windows API overview, select
Diagnistics->Performance Counters or Performance Logs and Alerts.
Windows SDK 7.0 has a sample in ...\Samples\WinBase\WinNT\PerfLogsAlert:
"This particular sample code collects the following data:
- Counter data: Processor(_Total)% Processor Time, Processor(_Total)% User Time"