Why does DWM.EXE use so much memory?

DWM.EXE is the Desktop Window Manager in Windows Vista. 

When you're using Aero Glass in Vista, DWM owns an offscreen bitmap for every open window, the size of the window, with a rendering of that window's contents.

DWM needs this so it can quickly show you thumbnails of all the open windows, and so it can do things like composing and animating quickly.  But it requires a lot of memory.

On my system, for example, opening a single full-screen Notepad window causes DWM.EXE to allocate an additional 4.8mb of memory.

Close Notepad and DWM gives this memory back. 

I opened a hundred Notepad windows to test, and yep, it allocated almost half a gig of RAM for the bitmaps.

If you can't afford this much RAM, don't use the Aero Glass theme in Vista.