I'm having problems with XP Proffesionl, basically my serial key was invalid (I'm sure your sick of hearing of problems with invalid serial keys and such and I'm sorry ) I wanted to install SP2 so I searched for a solution and found a registry file that apparently edited my current XP serial number. I used the file and changed the serial key and much to my delight SP2 installed just fine. It then asked me to restart my PC for the changes to take effect but that's when the problem arose. My PC started up as normal but as I try and log in I'm greeted with an error message reading; "A problem is preventing Windows from accurately checking the license for this computer. Error Code: 0x80004005" and windows automatically logs me off and takes me back to the login screen. I decided the best bet to fix this would be to use my installation CD to repair my current installation, if not completely reinstall XP but this is where I'm getting really confused. I put in the installation CD I used before but my computer starts up the old installation without asking if I want to reinstall/repair Windows which I'm sure it automatically did before. I tried a friend's XP installation CD as well just to be sure it wasn't a problem with the disc but again it goes straight into the old installation. I tried editing the setting in my machines BIOS to put my CD drive to a higher priority than the HDD and it then asks if I'd like to boot from CD but upon agreeing to this it simply goes back into the old installation again. I'm really stuck now so if anyone knows why my PC is failing to boot up the installation CD or has any other solution to the problem it would be greatly appreciated, sorry for the trouble and thanks for your time.