You have the right service pack,what you may have seen was the service pack sp1a which i believe is not necessary if you have WINDOWS XP, '5.1.2600 Service Pack 1 Build 2600.
If you are not running Windows XP SP1, install SP1a to make sure that you have the latest security and reliability updates for Windows XP. Install any appropriate security updates and hotfixes that are already released or that will be released after SP1 on Windows XP-based computers that are running SP1a. Windows Update will continue to offer the same set of updates for Windows XP-based computers that are running either SP1 or SP1a.
To determine if you are running Windows XP SP1, right-click My Computer, and then click Properties. If "Service Pack 1" appears under System, you are already running Windows XP SP1. There is no benefit to installing SP1a if you are already running Windows XP SP1. Microsoft does not recommend that you install SP1a if you are already running SP1.
Windows XP SP1a is the same as Windows XP SP1 except that the Microsoft virtual machine (VM) is removed in SP1a.