I have a new MSI P4M900M3-L Motherboard with 390watts ACBEL True power supply.

I attached to a 17" Philip 170s lcd screen, when I turn on the cpu, the lcd just show "no video output"....

I waited for about 10 mins and the the xp desktop just appear!

So what could be wrong?

390Watts of power is TOOOO LOW for your motherboard .....
Maybe impossible to run a C2D E6600 and an extra gfx card on that PSU

Get at least a 550watts PSU

Calling your computer a "CPU" is not acceptable in this forum. The CPU attaches to the motherboard. Speaking of which, your board choice wasn't a great one. Nothing wrong with MSI, but why get a board based on a VIA chipset? Intel based boards (P965 or P35) perform much better. They may cost a little more, but it would have been money well spent.

http://www.msicomputer.com/product/...

AcBel units get decent reviews. Besides, the PSU has nothing to do with the monitor. If the following is your unit, it has 16A on the +12v1 rail (for CPU) & +18A on the +12v2 rail (for the rest of the hardware). Your 390W PSU *should* be fine unless you plan on running a high end video card & multiple HDDs.

http://www.acbel.com/PDF/P_PC/e2%20...

Is this a new build with a fresh install of XP? If not, did you do a repair of XP after you swapped boards?

It is taking a while for your monitor and video adapter to sync would be my guess. Dunno why. I doubt it is to do with the power supply. It has little to do with video.

I read it wrong and answer it wrong too. So get off my case you peanut.