Not to sound insulting, but I doubt that any OS just locks up all on it's own. How much RAM do you have? If the answer is less than 128M, then maybe a simple memory increase could fix the problem. How much unused space is on your HD? There should be at least 20%. Lastly, (already mentioned) what unnecessary programs are running in your sys tray. Use msconfig to turn these off until needed. Ah yes, one more thing (dreaded) when was the last time you reformatted the HD. A yearly reformat is considered normal maintenance by many.
Hope that helped.