I had to replace the mobo/CPU/RAM on a computer running XP MCE 2005, and figured I'd have to reactivate. So, I booted into Safe Mode w/ Command Prompt to install the hardware drivers, which went fine. Then I rebooted, and I saw my friend, Mr Activation Wizard. I figured I could go ahead since the Network driver was installed, as were the rest. But, the activation window is just blank blue, with the "missing picture" icon from Internet Explorer in the upper right and upper left.
I knew in the past I could usually install Internet Explorer 8 in this instance, and it would bring up the activation wizard properly. So, I did that in Safe Mode w/ Command Prompt again, IE8 is installed, so I restart. Same thing. I end up re-arming the activation (gives me 30 days), and went to Google. Google sent me to Microsoft's page, which included checking the .htm registry entry in HKCR, running SFC /scannow (which didn't fix anything), and registering vbscript.dll and jscript.dll. I tried restarting and activating, but it is still blank.
slmgr /ipk <licensekey>
(to set the license number) followed byslmgr /ato
(to perform online activation).