Windows XP contains several tweakable memory settings in the registry, one of which is the DisablePagingExecutive registry key. This controls whether the operating system will transfer its essential driver and kernel files to the ‘virtual memory’ (the page file on the hard disk). It defaults to allowing this.
Obviously, transferring portions of the system to hard drive memory can considerably slow things down, and it appears that Windows XP does this periodically, whether or not the system is actually low on physical memory (RAM). If you have 256MB of system memory or more, try this registry tweak to force Windows to keep its operating data in main memory
Procedure to follow
Click on Start—>Run type regedit click ok
Navigate the following key from left pane
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager\Memory Management.
From the right pane double click on DisablePagingExecutive key change the Value from 0 to 1