Heres the hardware...
 
Motherboard: ECS AMD690GM-M2 AM2 AMD 690G Micro ATX AMD Motherboard  
CPU: AMD Athlon 64 X2 4000+ Brisbane 2.1GHz Socket AM2 65W Dual-Core
RAM: CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
Power Supply: Thermaltake Purepower 420W  
 
When I power up the computer, all the fans turn on but the system will not boot. I hear no beeps and the power button will not turn the system off, I've tried both on board (DVI and VGA port) and add-on graphics cards and do not get any picture. I've checked the CMOS reset jumper, and CPU fan power plug.  
 
Is it a bad mobo or ram compatibility? I've tried the ram in another system and it works fine.

i believe the motherboard is not compatible with your ram. go to corsair's website and see if it is compatible.

Thanks for the reply quanger.
 
I checked the site, my ram was not listed but this one was:
 
TWIN2X4096-6400C5 DDR2-800 (XMS2-6400C5*4GB) 4GB Kit (2 x 2GB) 1.9v  
 
and heres my ram:
 
TWIN2X2048-6400C4 DDR2 800 (PC2 6400)  2GB Kit (2 x 1GB) 2.1v  
 
 
Sorry for my stupidly (I probably shouldn't build PCs) but is it the voltages that could be the problem or what?
I really don't know what to check I guess when it comes to compatibly of ram for mobos, sorry.

Hi NerdWithNoName, I had this same problem. It turned out that I did not have the auxiliary power supply (the 4-pin one) plugged into the motherboard. So if you've done that, this may have to do with voltages.

I checked every connection on the board a few times, but it turns out my "first" problem was the PSU switch on the back of the case was set to the non us voltage setting! (why is it always the simplest thing for me?)  
 
After that it would still not boot-up but with an added beeping sound that points to a memory problem. So I took one ram stick out and it boots up! Why is that?? If I put it back in it starts to beep again. The ram is rated 2.1v the mobo is I think rated 1.9v, is this the problem?
 
Thanks for you help guys (and or girls)