Does the computer have a sticker on it anywhere with a 25-character alpha numeric code?  What operating system does it say?  You can also go to support.dell.com, enter your Service Tag, click on Warranty Status (left menu), and Original Configuration tab to see what it originally shipped with.
I purchased a Dell Dimension 5150/E510 from an ebay seller, came preinstalled with Windows 7 Ultimate. After a week of updates, get the message that Windows 7 is not Genuine.  I contacted the seller, he claims the copy is genuine, that he upgraded from Windows XP, sent me a CD to reinstall, long story short, this did not work and I am sure it must be a counterfiet copy. I need to purchase a legitimate copy. How can I tell what the original operating system was? If it did come originally with Windows XP, can I purchase a upgrade to Windows 7 or do I need to purchase the full version?   Thanks for your help.
How to find out what original operating systems was