Well first off what is the dwm.exe file anyway?
The dwm.exe is the Desktop Window Manager. It's what gives you thoughs really nice effects in vista like the thumbnail on the live taskbar thumbnails, transparent windows, and the Flip3D Switcher.
Now with that being said switching to the vista basic theme will help dramatically, but ti won't turn off DWM.
If you want to switch to vista basic head to Control Panel\Appearance and Personalization\Personalization and click on Window Color and Appearance and you can click on Vista Basic.
How Do I Turn dwm.exe off?
The only way to get rid of the dwm.exe process is to stop it from running. In order to do that head over to Control Panel\System and Maintenance\Administrative Tools and click on Services.
Now look for "Desktop Window Manager Session Manager" in the list, and click the stop button.
NOTE: Once you've done this you'll be switched to teh Vista basic Theme.
If you wish to permanently disable it, just double click it, and select Disabled from the "Startup type" dropdown menu.