Over Range But Was Fine Before?

My brothers computer recently developed a problem, its an old Dell optiplex GX1 500L+ and hes on XP pro
he came home and the monitor was flashing the little lights on the front that indicate it being over range
we tried an older monitor, as well as my own LCD that Im typeing on now, both had the same results, as well as in VGA mode the monitor acts normal up untill the loading windows screen, where it promptly goes blank and begins with the flashing lights but booting into safe Does work and gives us a picture, so I dont think it could be problem a problem with onboard graphics, so Im guessing its a malicious program that is inflicting this problem, unless someone can tell me otherwize does anybody know what would likely be the cause of this, or the best way to trouble shoot it, we already did a Repair installation,that didnt fix anything and I figured Id ask here on the tech forum before proceding to wipe his hard drive, my question on the forum is also a lilttle bit curiosity driven,