Excel 2007 severe slowdown

Running Office 2007, and specifically Excel, I notice that there are occasions when the program seems to drag on when shutting down. I can alway get it to happen if I have multiple spreadsheets open - especially if there are formula dependencies between them. A similar effect occurs if I attempt to modify a chart that was originally built in an earlier version of Excel ... The worksheet hazes over and a "not responding" message appears in the title bar .. It eventually clears up ... but can take up to 10 minutes.

anyone have any ideas what's going on with this? Thanks in advance!