4Gb not reporting correctly (Not same as other posts)

I have 4gb ddr400 installed on an Asrock 939 dualsata board. I dual boot
windows xp64 ,4gb reported correctly as it should.
Boot into Vista Home Prem 32 only 2GB shown, Not the expected 3.1gb or 3.5gb
than you would expect taking the memory addressing limitations into
consideration.

Only expansion card on system is PCI-e Radeon 3870HD, Realtek on board sound.
4 x 1gb sticks in use. Bios reporting correctly, No option in Bios on
current revision for memory hole, There is on the later revision of the bios
to enable/disable memory hole but when set to enable. Ram in XP 64 reports
2.5gb so actually drops the reported ram. Windows vista it will raise it to
2.5gb.

Driving me up the wall and idea's