SERVICE PACKS normally fix things, at least in theory. But changes to Windows XP's memory management system in XP Service Pack 1 can have the opposite effect. Thankfully there is a patch.
The problem comes about because of the way that the memory management system deals with memory allocation. If a program allocates and deallocates large blocks of memory too often that will cause Windows XP SP1 to snarl up.

However, although there is a fix, you shouldn't apply it unless you're definitely having this problem. Microsoft is so sure about you not installing the patch unless it's absolutely necessary that it hasn't even made the patch available on its site. You have to phone the company up and hope that it agrees that you need the patch.

Microsoft Knowledge Base Article - 815411