Boots to black screen with cursor

[kip360td]

system worked fine untill son decided the case needed a new fan, added fan using power from motherboard (mobo is ASUS P5wd2-E premium) when re-booted computer it would post then drop to black screen with curser. What I found was he also plugged power into the ez-connector, I'm not sure this would have any effect but it wasn't used as the power supply had the AT connector. I tried all the tests I knew but couldn't boot to F8, to try to do system restore, was able to boot to bios and select boot drive (cd) and tried to do a windows repair, (boot ini repair) the system sees drives, drives act as if it's cycling drive light blinks on case,did the repair but will not boot. anyone have any ideas