Ok i asked a similar question before, but the popular advice seems to be to delete the windows partition then start over if you want to reinstall windows on a mac. BUT im pretty sure windows does not allow the same copy to be authenticated twice, below is copy and pasted from the windows site: How do I activate Windows? ■Click Start, point to All Programs, point to Accessories, point to System Tools, and then click Activate Windows. The activation wizard will walk you through the steps required to activate Windows. ■For additional instructions, or if you have problems activating Windows, please see knowledge base article # 307890. What if I am unable to activate Windows? If you are unable to activate Windows by following the steps above or using the methods in the knowledge base article, the copy of Windows on your computer may be counterfeit or may have been installed incorrectly. What could prevent activation? ■If your computer is new, the copy of Windows installed on your computer may be counterfeit or may have been incorrectly installed. ■If Windows was installed or upgraded on an existing computer: ◦The copy of Windows you installed on your computer may be counterfeit. ◦A third party, such as a repair shop, may have installed Windows incorrectly or used a counterfeit copy. Anybody experienced this problem?