Hangs When Closing Programs

My father gets all the weirdest and most interesting problems with his computer... and then, of course, he turns to me for help, and usually it's something I've never heard of before, and probably caused by some random software gizmo he downloaded, that I just tell him to format and reinstall.

This time, though, it's on a fairly fresh (about a month and a half old) reinstall, and he says he hasn't installed any new software lately. So I thought I'd ask for a bit of help before telling him to reformat.

when he closes a program, about half the time it hangs for 10-30 seconds before closing. The program window disappears, but is still in the taskbar, and nothing at all can be done (no buttons will click, no programs will open, no keys will press) until it disappears. Afterward, if, for instance, you've clicked every icon on the desktop in frustration, most of what you tried to do during this brief hang will all happen at once. The Task Manager's performance tab doesn't show any unusual activity during the hang. And even the Task Manager will cause this hang when closed.