Windows Defender cannot be installed on Windows XP x64 Edition

My department decided to upgrade my old development pc and here are the specs:

Dell Precision T3500

CPU: Intel Xeon W3520 @ 2.67GHz

RAM: 6 GB

Monitors: 2 X 20’ flat panels

Hard drive: 300 GB

OS: Microsoft Windows XP Professional x64 Edition

The reason we are still using Windows XP is that my department is not convinced enough by Windows Vista and would like to upgrade directly to Windows 7 when it becomes available. Anyway, with 6 GB RAM on Windows XP, the computer runs very well but it also brings a lot of new adventures as far as the program installation is concerned, even with some programs developed by Microsoft. Here is a frustrating one:

Windows Defender x64 version cannot be installed on my pc and keeps giving this error:

Service ‘Windows Defender’ (WinDefend) failed to start. Verify that you have sufficient privilege to start system service.

The error happens at this stage of the installation. Below is the screen shot of the error message:

If I click “Retry”, it keeps giving me the above error; but if I click “Cancel”, the installation rolls back.

My account is an Administrator account and it should have sufficient privilege, so the error must reside in a different place, but I just cannot figure out where. If you had the same problem and got the answer, I would appreciate it if you can tell me how to fix it.