Hi,
Although Windows is reporting that only 3 Gb is usable, are you seeing all 4 Gb's during POST when you first power up your PC?.
A quick glance through the manual for your Mobo doesn't show any settings that you can make in BIOS they all seem to be read only when it comes to memory so, I don't think there is anything you can do there.
Windows always takes a certain amount of RAM for it's own uses. Also, if you are using On-Board graphics rather than a dedicated card, then I think the system will take some RAM for this (not certain about this on Intel but it certainly does on AMD systems and maybe Dark Mantis will correct this if I am mistaken) which maybe the reason you can only use 3Gb's.