Some information straight from Microsoft:

When you start the Windows Product Activation (WPA) Wizard, you may receive a blank "Activate Windows" page, and you may be unable to continue with the activation process.
This issue can occur for any of the following reasons:
Your registry is damaged or has been modified.
Files became damaged during the Windows XP installation, or files are missing.
You have a video resolution problem.

To resolve this issue read the whole thing from here:;en-us;Q314935