Problem with start up

Hello,
im just wondering if there is some one that can help me i have looked at every thing and still have no idea
the problem is when i start up my pc all the lights and fans come on but it restarts its self before it boots i thought it might be a virese so i disconected all the HDDs oh yeah i got some data off a mate and thats when it started and now my external HDD wont even show up in computer management what ever has caused this it has also done it to my other pc when i pluged the HDD in to it and when i plug the HDD into my laptop it makes the laptop take for ever to start up thanks