Windows 7 running slowly with low CPU usage and memory

It seems like it could be the hard drive dying, which would slow windows down.

These symptoms sound like a CPU throttling issue that I had. When my laptop overheated, the CPU would reduce its speed (throttle back) to reduce heat output... as a result my CPU % appeared to be low at around 15%, but in fact the CPU had cycled back to 16% of its max and so it was in fact running flat out.

You can diagnose this by using Resource Monitor instead of Task Manager. (Type "resmon" into the Start-Run prompt.) At the top of the CPU tab processes window there is a useful "Maximum frequency" which will reduce as the CPU throttles back.

If this is the problem then you can look at causes of the throttling. You can use free tools like Speedfan to monitor temperature and check on your fans.

In my case there was a definite correlation between throttling and temperature. My laptop was a Dell E6400 and I used a fan assisted cooling tray to assist with the overheating, and I also hit a known issue with this laptop model - see http://www.xtremesystems.org/forums/showthread.php?240717-Dell-Latitude-E6500-E6400-ThrottleGate-Fix

This might be caused by the selected Power Plan. The "Balanced" power plan recommended by Windows is not very clever and can affect the performance badly (50% in my case).

So: check the Maximum Frequency in the Resource Monitor, like SteveW explained. If the Maximum Frequency is not 100% check the Power Plan (battery icon in your tray bar).

Set the Power Plan to High Performance. If it solves the problem then you can start playing around with other plans or configurations but at least you have identified the issue.