This problem may occur if the programs you run frequently allocate and deallocate large blocks of memory. Changes in Windows XP SP1 in the memory management system have caused this operation to take significantly longer than with pre-SP1 Windows XP.

A supported fix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Apply it only to computers that are experiencing this specific problem. This fix may receive additional testing. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next Windows XP service pack that contains this fix. The hotfix is not publicly available.

Programs Run Slower After You Install Windows XP SP-1

Thanks to NTFS.org.