Erik is right, 3.5 out of 4 GB being used by Windows/ various background processes doesn't sound right. 4GB should be plenty for simply running Skyrim without many other programs being open at the same time. Another thing to consider is that Windows caches some applications that you use often into memory so that they startup quicker. That memory is reserved but not technically in use and will be released to the program that needs it (Skyrim in your case) if necessary. Still, you should have more than 0.5 out of 4 GB of free memory when you are not doing much on the PC.