Pc Wont Turn on

[MDOT]

Hey,
I'm repairin a friend's computer however i have ran into a problem that i have never incountered before
When i try to turn on his pc, nothing turned except his opitcal Mouse (ps/2) which lights up red,
i tried one of my power supplies and the pc still wouldnt turn on only the mouse.
His story is "he thought he had a virus so he replaced his hdd with other harddrive he had laying around which didnt work. however when he connected his original hdd the pc doesnt power up at all, only the mouse's red laser.
no fans are turned on and no beeps for the system.

from what i can see inside the computer, he has a 80 gig hdd with 512 mb running windows xp.

[patio]

Disconnect everything except for CPU, RAM, vid card, keyboard and mouse and power up and see what happens...

patio.  8-)

[MDOT]

Hey patio thanks for the reply
I disconnected everything u told me to, however on the mouse and keyboard lights turned on, still no fans

[patio]

Either that power supply is shot or that motherboard has gone south on you.
Try a brand new power supply using the same setup...

patio.  8-)

[MDOT]

i guess its the motherboard like i suspected
now i have to find him a replacement, hopefully the same model!
Thanks Patio

[Raptor]

Replace the CMOS battery and see if it still doesn't power on.

[MDOT]

Raptor, i will check again tommor