Beginning with Windows XP, Microsoft put into place a verification system meant to reduce the amount of piracy of the Windows XP Operating System (OS). Windows Product Activation (WPA) requires users to verify their OS, and if verification fails, the OS ceases to work. Unfortunately, some XP users have discovered their OS being mistakenly rendered useless by WPA, which is why you may be seeking to bypass WPA. Bypassing WPA, while not an easy process, is actually possible by editing a registry entry.
Click the "Start" button, then click "Run" and type "regedit." Click "OK" and the Registry Editor will appear.
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Scroll through the list of entries and find "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\WPAEvents." Click it once to see what is inside the entry.
Select "OOBETimer" within the entry and change the value to "FF D5 71 D6 8B 6A 8D 6F D5 33 93 FD." Hit "OK" to confirm the change.
Right-click "WPAEvents" and select "Permissions." Then click "System" and "Permissions for System."
Click all the boxes under the "Deny" field so that they are all checked. This prevents Windows from re-activating WPA.
Click "OK" and then exit Registry Editor. WPA will now be disabled. If it is not, repeat Steps 1 to 4 to re-disable it, which Windows will not be able to reverse now that you've denied the system privileges to change the values.
Consider contacting Microsoft first before bypassing SP2 WPA activation. In many cases, Microsoft will attempt to help you in verifying your copy of Windows XP so that you don't have to deal with WPA anymore.