Please review the systems quick spec sheet I posted above to locate what your exact model is.
Most of this series is not able to run Windows 8. You might be able to get one of the Core 2 Duo based systems but you'll need to upgrade your memory & graphics board.
Review this MS Windows Upgrade help FAQ: [http://windows.microsoft.com/en-us/windows-8/upgrade-from-windows-vista-xp-tutorial|Upgrade to Windows 8.1 from Windows Vista or Windows XP]. Make sure you check your hardware fully.
Given the cost of a new system with Windows 7 or 8 already installed you may want to think about getting a new system Vs trying to upgrade this system to either Windows 7 or 8.