I think his problem is he is using windows 7, has 4GBs of ram, but only 2.87 is usable. But by not giving us any system specs, we can only guess. I'm going to guess he is using CF/SLI and has 2 or more large ram video cards in use. The answer to this "problem" is to upgrade to the 64bit copy of windows. This won't cost you anything with Win7.
The OP didn't say he was using 64-bit Windows, only Win7. And not all copies of Windows 7 include the 64-bit version. The solutions offered in the other forum may apply to him, chking remapping of RAM, etc... But first is to verify he's using 64-bit
Shubham, your problem is different and relatively simple. Gigabyte already explained it to you, perhaps not clearly. For reasons similar to why a 32-bit OS can't use all 4GBs of RAM, a system with a maximum of 4GBs of RAM cannot use it either. It is limited to an addressable space of 4GBs therefore many system processes need to use those addresses - in addition to RAM.
On the motherboard specs page: 2 x 1.8V DDR2 DIMM sockets supporting up to 4 GB of system memory
* 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.
If this board supported 8GBs of RAM, you'd be able to use the full 4GBs less whatever is reserved for the onboard graphics - I can't find out how much this is but perhaps as much as 512MBs but most likely 256MBs.
(Since you're registered at the other forum, you might post this answer there also)