mowpar
Your situation here is totally different than us helping you with bypassing BIOS/security passwords, we are aware of what has brought you to this point and rest assured that we would not have offered assistance if there was any doubt about the ownership of the computer or OS.
Have you tried leaving the user name and password fields blank, if you have done a completely fresh installation you should be asked to create a new user name and password not to enter previously used information, this would suggest a repair to a presently installed OS, odd but not unbelievable with a Dell computer I`m afraid.
Can you tell us what steps you followed when installing Windows 7, did you fully NTFS format the HDD for example, were you asked where you wanted to install Windows 7, this to see if there are other partitions on the HDD.
Do you have the 25 digit product key for Windows, there should be a sticker on the case either on the side or the rear like the in the attachment below, we do not need the COA information if there is only confirmation that it is there.
If you do have your product key I suggest a full and clean install of Windows 7 using a downloaded ISO which we will help you with if appropriate.