My ConRoe945G DVI Motherboard has 4GB of RAM installed but it only registers 3GB

According to the specs:

Memory
- Dual Channel DDR2 memory technology
- 4 x DDR2 DIMM slots
- Supports DDRII 667/533 non-ECC, un-buffered memory
- Max. capacity of system memory: 4GB*

*Due to the chipset limitation, the actual memory size may be less than 4GB for the reservation for system usage under Windows® XP, Windows® XP 64-bit, Windows® Vista™ and Windows® Vista™ 64-bit.

There's absolutely nothing that you can do to resolve the issue because memory remapping isn't available on that motherboard.
 
As mentioned Windows XP and Vista will only recognise 3gb normally. Windows 7 has made the difference as it can actually use more than 3gb of memory.
 

If you read the original post, the issue is at POST. ASRock state in the specs that it's a chipset limitation even with XP 64-bit and Vista 64-bit.