Can't load windows

Hello,

new computer, 1tb hd, diamond hd4870, asus p5q3 deluxe, had installed windows but was having some weird problems , so decided to reinstall windows , formatted then installed but after windows loads the first set of files then reboots , windows goes back to the first screen again to load the files, cant get it to finish loading

two days ago windows loaded fine.. if anyone can help would appreciate it thanks