i found a Tower Computer an Athalon something , has 3 g mem , 3 ghz speed quad cpu says it was made for Vista but i wanna put XP Pro on it ,

can't get Video to come on , tried a few things like switching mem cards there's 4 of em , tried 2 matching ones only but still no video , tried disconnecting the Sata Hard Drive in case it was interfering still no video , i connected an Ascii Hard Drive instead of the Sata Hard Drive still no video , tried 2 separate video cards one PCI and the other one a brown slotted Video Card but still no video , scratches head , what could be the problem ? , the fans still function 2 of em so the machine is getting power