My computer will start up for approx. 3 seconds then turn off. If you push the start button afterward it will not start at all. If you remove the power cable going to the motherboard from the powersupply and then reconnect it, it will start up again but only for those 3 seconds. I think it is something to do with the motherboard just not sure what. Do i need a new mb or is there something i can do?

You need to re-install windows from scratch. It happened on my computer once.I dont know what caused it,i just know it works.

You have an electrical short in one of the devices. Try disconnecting the power to everything except the motherboard. If it still fails, pull all the cards. If it still fails, you have a bad motherboard.

Try clearing the CMOS. Disconnect all power form the machine. Then drain the board by pressing the power button. Then remove the Motherboard battery for about one minute. Then replace the battery. Reconnect the power and then try to start up the machine. Does it Start?

"You need to re-install windows from scratch." Kind of hard to do that if the system won't start. jim is on the right track. ensure you check any standoffs that the mobo is sitting on, especially metal ones with securing screws going through the mobo. If there are any lock-washers on these, check to make sure they're not touching any tracks in the mobo. (There's usually fibre washers on these type mounts.) Also check for minute stress cracks around mounting/support points. (Pushing cards into slots around unsupported areas of the board can cause these.) You may have a track that can be repaired with careful soldering. And as jim says, if it still fails, go shopping.