Windows 7 x64: 3900mb of 4gb RAM usage, even when (almost) nothing running
Solution 1:
You need to check a number of things:
Run the Task Manager, switch to the "Processes" tab and check the "Show processes from all users". Then when you sort by CPU usage is there anything that stands out as anomalous? Do this with as little as possible running (no Explorer window or web browser etc.).
Run a virus/adware scan. You say you don't think it is one of these, but it pays to rule it out for definite.
Run a memory scan. This should show that everything is OK, but again it will rule out memory problems.
Think back - was there anything (no matter how trivial) that changed two or three days ago. If you installed anything, uninstall it and see if that helps. If you have a restore point from before the problems started roll back to that and see if that cures the problem.
If the problem still persists then I don't know what it could be, but you'll have a lot more information which you should add to the question so someone else might be able to help.