In most cases you can't go back to Vista, unless you have the Dell install discs.  Once a new OS is installed on a PC, it usually corrupts the Dell PC Restore (recovery partition) on the hard drive.  However, give it a try just in case it still works.  F8 at power on will access the Dell PC Restore option, but DO NOT run the PC Restore (if it works) until you have backed up your user data as it will be lost in the reinstallation. If the PC Restore will not work and you have the Vista install disc and the Dell drivers and utilities discs you can do a manual reinstall and then using the Win 7 upgrade disc upgrade to Windows 7.     But, before you undertake this you need to backup up your user data to some external media - either discs, an external hard drive, memory "key", etc as all will be lost when you reformat and install.   HERE is the Dell install instructions for both PC Restore and the manual procedure.
Hi, I bought the Dell Inspiron in July last year. As part of this I became a valid candidate for the Windows 7 free upgrade by Dell. I upgraded the computer and it is running smoothly. However, my computer is getting rather cluttered up, and strange things are happening which I can't explain. (It no longer automatically connects to my router, even though the check box for this is ticked) I want to restore my computer to default factory settings, but can't because Windows hasn't saved the old OS. I still have the Windows 7 upgrade disk. Should I try to restore the operating system using that? If yes, how do I do it? If not, then how am I supposed to restore the computer.    Thanks in Advance.   P.S. I don't mind reverting back to Vista then upgrading again, if that's what it takes.
Windows 7 Recovery