12GB RAM(3,95 available)
Windows 64-bit, in MS config the checkbox was removed, even tried to rearrange the slots. Nothing helped. There are options to solve the problem?Is a PC or laptop ? What PC or laptop ? Maybe he's old and doesn't support more than 4 GB of memory (although it's hard to imagine so older PCs, but you never know). Windows sure 64-bit ? And you never know - put like 64, and in fact will 32. Well, then - where'd you get that 12 GB if it doesn't work ? She worked before ? Memory from the store or b\y ? Actually, there infa that the memory is working ? In the end, try in turn to put the memory and test that each strap separately, PC was started (if already done - write result).
Try in the BIOS to dugrenot the frequency of the RAM, and see if it will work.
Or trite strap can be reversed to change, I once helped.