*** I think that is a P4.? You may want to check your cpu temp. P4 and P3 (More so P4) will run slower at high temps to cool themselves off. I ran into this with my new P4. I had a bad thermal pad on my heatsink. I removed the pad and used arctic silver thermal paste and it corrected itself. This will cause pauses in processor intensive programs. My 3D screensaver (had about 50 ships battling) would even lag my 2.8GHZ P4. Now it will run anything fine. Make sure your case is clean of dust also it will cause all kinds of sluggishness.
*** If that doesn't work I would look into driver updates. You can right click start,programs,accessaries,system tools,system information, tools, then DirectX Diagnostic tool to check and see if there are any driver compatability problems with DirectX. You should have DX 9.0b installed.
*** Also you may want to make sure no one has turned off your systems pagefile. Right click on my computer, click properties, advanced and under performance click settings, then click advanced tab, under virtual memory click change. At the bottom it will tell you what the recommended for your system is. Make sure it is at least that or just click on system managed size. Click ok and it will bring you back to the Performance options advanced section and make sure both processor and memory usage are set to programs instead of background services or system cache.
Hope this helps I have to sleep now
Good Luck!