Tricky one, this. It almost sounded like you may have had a nasty piece of spyware/adware/nastyware running, but you've disabled everything from startup in msconfig (not always the best thing to do, especially if you rely - as you should - on AV software like Norton AntiVirus).
I would suggest downloading SiSoft Sandra from here. This will allow you to benchmark many aspects of your system including your memory and CPU. If the CPU score comes out very badly, then there is something wrong here.
My primary concern is that you could have an over heating problem. In the event of over heating, the P4 ramps down it's performance to reduce heat, which can have a devastating effect on games, like you're describing.
This overheating will probably be due to a defective or improperly connected heat sink or fan.
I would therefore suggest the following courses of action:
1) Open Task Manager and watch your CPU activity when the machine is freshly booted. After everything has settled down, your CPU should be more or less idle (0 - 5% fluctuation). If it's permanently maxed out or very nearly, then some software (possibly viral) is eating your clock cycles (look in the Processes tab in Task Manager, see what is using a lot of CPU %).
2) When you load up a game, switch back to the Task Manager and see if another process has loaded and is eating CPU power then (other than your game!)
3) Download mbprobe from here and also the NT support file from here. "This archive is required if you want to use MBProbe under Windows NT/2000/XP. The installation instructions are included in the main MBProbe archive- please follow them to the letter".
Once working, configure it to only show the CPU temp in the system tray (by default it will blink between CPU/Motherboard temps) and uncheck anything which may be warning you of bad voltage readings (mine are mental, but the system is rock solid and very fast) except for the CPU itself. If the CPU itself is majorly overpowered, then that's why you're overheating.
Next, put a copy mbprobe.exe into your startup folder, shutdown your PC and leave it for an hour or so to cool down.
When you next boot into Windows, keep a watchful eye on the temperature gauge. With luck, the system will not heat up and switch to low power mode before mbprobe loads, so that you can watch it go nuts and then drop down again as things slow down.
With a P4 system the CPU's attempt at self preservation seems the most likely conclusion.
Hope this helps.
