IE 7 is well known to seemingly random closings. Since we can't fix that we try out other browsers like firefox. Sometimes I find changing NIS 2008 to another security suite to help but all that depends on how amiable the owner is to change. Sometimes we have to wait for the owner to stew on the issue before they'll accept the changes.
I have both a desktop(Dell Dimension 4500) that's hooked up via a wireless router to my laptop (Dell Latitude D600) and in the last few months, when I'm working on the laptop, (for example) I'll have my email program working(but minimized), a Yahoo program (again, minimized) and I'll be actively working on either another website or doing a virus scan (NIS 2008) and all the sudden, the minimized programs will shut down without warning and without closing them myself. I'm running IE 7 as my operating system. I've never had this happen while I was using the desktop, but since I'm disabled and it's easier for me to use the laptop, it's a real problem to have my open sites close down for what seems to be no reason at all.