From Microsoft:

If you are not running Windows XP SP1, install SP1a for security and reliability updates to the Windows XP family of operating systems. There is no benefit to installing SP1a if you are already running Windows XP SP1. We do not recommend that you install SP1a if you are already running SP1. Windows XP SP1a is almost identical to Windows XP SP1 except that the Microsoft virtual machine (VM) is removed in SP1a.

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