This is commonly caused by a bad or corrupt video driver or system memory, but since this is an eMachines, anything goes. How old is this "computer"?
Have you installed all the Windows 7 updates including Service Pack 1?
Check the hard drive for errors, by scheduling a check on the next start up...
Have you tried renistalling the video driver from the emachines D644 support website?
Have you tried renistalling the video driver from the emachines D644 support website?