The other day my windows machine became really slow. I rebooted, which had no effect. Looking in procexp, most processes were taking much more CPU then I'd expect, and I had a feeling the problem was the machine was getting stuck with a low clock multiplier.
To test the theory, I ran a tool called CPU-Z and sure enough it showed my CPU was getting the lowest possible bus multiplier.
(To fix the problem I pulled and re installed the battery, and then the CPU stopped being throttled).