[HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Control \ PriorityControl]
This setting effectively runs each instance of an application in its own process for significantly faster application performance and greater stability. This is extremely useful for users with stability problems, as it can isolate specific instances of a program so as not to bring down the entire application. And, it is particularly useful for users of Internet Explorer, for if a rogue web page crashes your browser window, it does not bring the other browser windows down with it. It has a similar effect on any software package where multiple instances might be running at once, such as Microsoft Word. The only problem is that this takes up significantly more memory, because such instances of a program cannot share information that is in active memory (many DLLs and such will have to be loaded into memory multiple times). Because of this, it is not recommended for anyone with less than 512 MB of RAM, unless they are running beta software (or have some other reason for needing the added stability).
There are two parts to this tweak. First is to optimize XP's priority control for the processes. Browse to HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Control \ PriorityControl and set the "Win32PrioritySeparation" DWORD to 38. Next, go into My Computer and under Tools, open the Folder Options menu. Select the View tab and check the "Launch folder windows in separate process" box. This setting actually forces each window into its own memory tread and gives it a separate process priority.