Activation
Click Start, click Run, type: regsvr32 regwizc
Press Ok
Repeat the same for the following:
regsvr32 licdll.dll
Blank Activate Windows Page in the Windows Product Activation Wizard
http://support.microsoft.com/KB/314935
HOW TO: Activate Windows XP
http://support.microsoft.com/kb/307890
How to activate Windows XP using an Unattend.txt file
http://support.microsoft.com/kb/291997
Windows XP Logs You Off When You Try to Activate the Product
http://support.microsoft.com/kb330136
For Vista activation issues see: Activation - Vista