Starting with Windows Vista and continuing on through Win 7, the OS uses quite a bit more RAM than older OS's such as XP. More complicated processes running, a fancier Interface, and more functionality require more resources. I would say with normal background services running Windows 7 will use between 1.5-2GB of RAM when it looks like it's "just sitting there." Opening up other programs will naturally require more RAM. You can always open up task manager, click on Processes, expand the window so you can see all the headings(Image Name, User Name, CPU, Memory, Description) click on the Memory header to organize it in a descending list, and see what is running and how much memory it is using to run it. Some things should look familiar such as firefox(or other browser), Skype if you use it, dwm, explorer, etc. For a more detailed list, while still in the Task Manager, click on Performance > Resource Monitor > Memory Tab. If you see anything that you really don't know what it is, Google it.
It is normal though, for a Windows 7 computer to be idling and using 1-2GB of RAM........at least on every computer with Win 7 I've encountered anyway.