K9N6SGM-V boot problems

Hi all, the system specs are:

K9N6SGM-V motherboard
Sempron AM2 3000+ 1.6mhz
2 x 512mb Ram from crucial
80gb pata hd
liton dvd-ram drive

windows xp home edition SP2 with all hot fixes up to 18/03/08

This was a budget build for a friend. I installed everything myself and I even ran stress tests on it with no problems.

Now the system does this. You turn on and it'll boot up and go into to windows, if you log out of windows/turn off then try to turn the computer on again it won't go past post. You have to unplug/turn power supply off for a few minutes then it'll boot up as normal. Any suggestions why it does this?

I've used a 20-pin casecom atx psu, I thought it might have a faulty powergood signal so I tried another 20-pin psu with higher voltage but still same problem. I've tried removing one ram module and dvd drive still no luck. I'm thinking it could be a faulty motherboard then why did it work fine when i first installed it? Should I use a 24pin psu??? Any help would be greatly appreciated as I'm now stumped.

Many Thanks in advance.