Usually, I go through the following steps to diagnose constant high CPU usage problem:

1. Open "Task Manager" to check which process(es) are having constant high CPU usage.

2. If the image name looks suspicious, I would enter the image name into Google to search to see it is one of known exectuables (e.g. not virus / worms).

3. If Step 2 turns out to be virus, search solutions for virus removal. Else, search "XXXXXXXXX.exe high cpu usage" in Google to find out the cause of the high CPU usage. Chances are that others would have similar problem.