Windows XP includes an option to utilize more memory for file system caching. Go to Start -> Run -> regedt32, find HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management, change LargeSystemCache from 0 to 1, press OK, and then exit.
The result should be increased performance in systems with a lot of memory; note that if you are low on memory, this tip will not help you out! In fact, this tip will hurt you, so make sure you have at least 512 MB of memory before attempting this. Also, setting DisablePagingExecutive can similarly boost system performance for systems with 512+ MB of memory.