XPC suddenly won't boot anymore

[humba]

I have a Shuttle SN41G2 XPC which has worked just fine until recently. About 3 weeks ago, I got a new screen, the Samsung SyncMaster 213T to replace my SyncMaster 191T. Since then, the XPC has been performing weird. Some time it would boot up, then freeze before I could log in, some time it would freeze during the day when I wasn't around, some time it would not boot at all (the fans and HD would spin up, but the big system fan would stay at full speed, never reducing rotation speed, and no picture would ever be shown on the screen (normally the main fan would spin up, then reduce RPM again, then the standard initialization screen would be visible on screen).
Today, I started the XPC, went to do something and when I came back, it had frozen at the login screen. I reset it, and since then, it will freeze at the point where the main fan rotates at full speed. There never is any picture on the screen, so the PC never makes it past post. The CPU led is green, and there are no bios beeps that would indicate any kind of an error so I'm at a loss of ideas. I have started removing add-in cards and non essential IDE devices but I still can't get past the point where the fan starts turning at normal speed and the BIOS displays CPU frequency, RAM and installed IDE devices.
Does anybody have an idea what could be wrong?

[Bunjiweb]

I'm sure it couldnt have been caused by using a new screen? that is unless your graphics is seriously overheating.

Do you use an AGB graphics card or onboard? if u use AGP have u slightly un-seated it from the AGP slot?

Can't think of anything else really.

Ben

[humba]

as I said, I have removed the AGP card so the on-board GFX should enter in action.. but that didn't do anything. In fact I have the computer in a bare status now, not even with a HD, and I just can't get past the full speed fan phase.
I'm pretty certain the screen hasn't had to do anything with this either, but since the problems started around that time and you never really know with computers I mentioned it. I've been assembling PCs for 12-13 years now but this is definitely the weirdest thing that has ever happened to me. I'm going to put the XPC HD into my secondary computer now to see if it is still OK. I also can't blame RAM or CPU because if the system would get around to the point where those are tested, and something was wrong, it would give me one of the beep codes.. I've had both ram and cpu failures before, and with the help of the MB manual I had no problem pinpointing the failing component.