You really haven't provided enough information to give an answer, so I'll guess.
Are you running Windows in a VM, like VMWare Fusion? The VM cannot utilize all your Mac's memory since enough memory has to be allocated to the Mac OS to run. In Fusion, you can set how much memory is used by Windows. Ditto for Parallels. But if you give it too much then memory swapping to disk will occur and everything will get very slow.