Ram issue with Asus board

[breadturbo]

Just bobbing a new mobo in a friends pc for them and i've come across an odd issue.
The machine shows 8gb installed ram but only 3.92gb usable even though it's running 64bit win7?

I've had a look for hardware reserved memory and can't see anything, anyone have any thoughts?

Spec is....

1055t @ stock
2x4gb corsair vengeance 1600mhz
Asus M5a97 le 2.0 (updated to latest bios)
6870

puzzled o.O

 

[breadturbo]

Ty for the help peeps

This gets more confusing, i've tried two sticks of my ram in it and it shows 8gb no problem in any slots, i even tried booting mine with just her two sticks, mine shows 8gb.
tried 1 stick at a time in all 4 slots and both of her sticks work fine solo.
The ram is on the mobo supported list, I didnt put the cpu in so i guess checking that is next port of call.