mainboard replacement went bad ...

Hi,
I usually replace mainboards on my pcs and have things turn fine, but this time, this mainboard replacement simply has no solution from my part: I replaced a mainboard because my system wont stop freezing (reinstalling windows was leading to nowhere and all devices such as hds, ram,floppy,video+sound+other adapters were tested fine on other pcs)..so I did replace that mainboard by a similar brand new one...I did plug back correctly all devices but when the PC boots, the screen stays blank! In normal circumstances, when that pc boots, the front leds (for Hard Drive and Power) flash ..then stop flashing, then the HD led would flash from times to times for usual data activity on the hard drive....but this time, when it boots, the two front leds
stay green ....
I dont think it has to do with my video card since I tried it on other pcs and it work fine...
PS: When the PC boots, there are no beeps at all..simply silent boot up with the monitor staying blank (the monitor is also working fine when connected to other pcs!)
any clue??

I have had this problem occasionally, the only thing I can narrow it down to is the power supply and that (if this possible) one of the many capacitors in the MB builds up a residual charge preventing corret working. I have to leave mine for about 2 hours or more before it will start again. This happened a lot before the first power supply went bang, now with the new one it has only happened about 3 times in 4 months. I am in the UK, 240v. Try a different power supply in your PC and see what happens.