CMT - if you are still experiencing system "freeze" or "lock ups" then I would suggest:
1. Check that your system isn't overheating. I have seen cases where the power supply cooling fan stopped working and a heat buildup will sometimes cause a system to stop running. Other cooling fans in your case may also have stopped e.g the video card fan. If you feel the outside of your PC case and it is very hot to the touch, you may have a cooling problem.
2. Run scandisk on your hard drive. This may take some time to complete but should detect logical and physical errors (and correct them if you select that option).
3. Do a complete virus scan of your system. Again, this will take some time to complete but is well worth the effort. I use AVG also and find it generally excellent. You need to be connected to the internet for the auto update feature of AVG to work correctly.
4. Check your driver levels. This might be an advanced topic, please reply here if you would like some more advicew with this one.
best of luck, I know that unexplainable lockups can be a horrible problem
oh, just a final thought, have you considered moving up to Windows XP?