To be quite honest, I hadn't thought of monitoring counters outside of SQL Server. You've sparked my interest.
We did add code to track disk space when we were trying to justify additional SAN space. The overall process is the same; simply capture today's disk utilization and subtract yesterday's from it to get a daily increase/decrease.
Take care,
Bert
"Speculations? I know nothing about speculations. I'm resting on certainties. I know that my Redeemer lives, and because He lives, I shall live also." - Michael Faraday