4GB memory seen as 3GB in BIOS

I have a 945GCM S2L Motherboard which has 4GB memory (2x2GB) running windows vista x64.

The RAM is getting reported as 3GB in the BIOS, Total Physical Memory also shows 3GB, Task manager also shows 3GB. Am I missing anything ?

EDIT: Is there any way of updating BIOS from a x64 Vista installation, 16bit applications are not supported by x64 & my manufacturer does not have a 64 bit install

Your motherboard does support 4gb of memory, with some caveats. Your onboard video will reserve some of that memory you have on there for its use, and you will also have some of the memory being used for system maintenance and such. After all this is said and done, you will only have 3 gb available for the os and any apps to use. This is the bad part about onboard video... shared memory allocation.

According to the product specification sheet here the board only supports 4GB of RAM. However the following is also listed

* Due to standard PC architecture, a certain amount of memory is reserved 
for system usage and therefore the actual memory size is less 
than the stated amount.

Maybe try and update the BIOS since sometimes this can help.

Edit: As already mentioned some of the memory can be reserved for the Video Card.

Maybe there is a shared memory used by graphics card? Some cards use main memory from computer in order to display contents of the screen.

Look for shared memory (or video memory) and make the value lower.