about 10 days ago the psu blew, it still powered the cpu fan, graphics card fan and case fans but the pc wouldn't start up ( i couldn't hear the hard drives) i bought a new psu but the pc started losing power every so often so i got a replacement psu but had the same problem
whenever it goes off i have to remove the plug on the power lead from the mains for a few seconds then it'll start back up, i'm certain that it's nothing to do with the mains though as i've tried different sockets in my house plus a m8's house also i don't have problems with anything else plugged into the same socket
it seems to go off whenever i'm using my dvd or cd drives, at first i thought it was my dvd burner as i ran the pc for a few hours with just the hard drives and it was ok, as soon as the dvd drive was powered it went off within 15 minutes of starting up, i tried the cd drive and it was ok for over 3 hours, i've now got a new dvd burner but still have the problem and it sometimes goes off when i'm using the cd drive

altogether i've tried the following
2 different psu's
2 different power leads
a spare m/b, cpu and ram that i've got
different mains sockets

my pc specs
kt3 ultra2 m/b
athlon xp 2100+
256mb ddr
120gb hdd
15gb hdd
ge-force 4 2200ti
pioneer 105 dvd-r
liteon 48x12x48 cd-r
network card
2x case fans

the psu is 400 watt so shouldn't have problems powering it

i've also checked for a virus but am 100% certain it's not that cos of the fact i have to remove the power lead to get it to start back up

sounds like a heat problem m8 especially if it only happens when the dvd drive goes on. install a system monitor proggie and keep an eye on the temp. also check the bios for the power settings to see what temp ur cpu is allowed to reach before it will shut down. if this does not work then get a bigger psu or an in case fan.

temperature seems ok to me, cpu is normally around 45 c and the chassis is around 33 c, the bios and my monitoring software are set to shut down at 70 c
i already have 2 case fans and have been using this pc for around a year with no problems

you could try a surge/spike protector or a UPS. I've had two PSUs blow up here, and since putting one in no problems.

But it doesn't sound like this is your problem. Have you tried it in someone else's house?

Apart from that i would guess that it's a temperature problem :( but as you've said that's not the problem!