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Can you log on in Safe Mode? If so try running the mgadiag utility from Safe Mode.

If the mgadiag won't run in Safe Mode, try re-arming Windows XP activation by following these directions:

Click Start>Run.

Type (or better yet cut and paste) the following command into Run and click OK.

rundll32.exe syssetup,SetupOobeBnk

[Note: command must be typed exacly as above with space, caps, and comma.]

This command is designed to re-arm Windows Activation to give you another 30-day grace period during which time you can make more activation attempts, doing a Telephonic activation if necessary.