There are a couple of limitations that Microsoft put in to Windows 7 upgrades that can prevent users from upgrading and ultimately force them to do a clean install. First limitation is that they won't let you to upgrade Windows 7 if you run Windows 7 release candidate that was released earlier this year, which they won't let you to upgrade to a download Windows edition. If you are running Windows vista ultimate you cannot upgrade windows 7 professional. We have to buy an ultimate edition then upgrade Windows 7 release candidate to Windows 7. Copy all the files from Windows 7 upgrade CD to your computer after open a folder sources open ini then change the Min client to 7000.0 after install Windows 7.