Check your IRQ's in the Device Manager (System Properties, Hardware, Device Manager, View Resources by connection). You will probably find that IRQ X is being shared by tons of devices. All the sharing can cause system slowdown because everything is competing to use that IRQ. It also causes soundcards to pop, videocards to stutter, etc.
Windows XP doesnt let you change these setting no matter what, even going through the bios cant change the IRQ's.
Changing the IRQ's is fairly simple though. Please Read with Care.
Warning! Doing this procedure will cause the power management to stop functioning, i.e. Standby and Hibernation. Also most drivers will need to be reinstalled. In my computer only the video card needed to be manually reinstalled. Please make a Restore Point if you wish to go back to orginal setting.
1. Go to System Properties
2. Go to Device Manager
3. Click the Computer + sign.
4. Find the ACPI Power feature.
5. Go into its properties.
6. Change its drivers to Standard PC.
7. Reboot
8. Drivers will reinstall, possible manual installation required.
9. Now devices should be more equally distributed throughout the system.
10. If IRQ's are still being shared, bios changes can now affect the IRQs.