excel closing - no error messages

HI, I have Office 2003; Recently, as in, the last few weeks, I'll have Excel open with a few xls spreadsheets; move to another application - IE, Outlook, Word - and I'll suddenly notice that Excel has completely closed - no error messages or anything.

Just now: I had a local worksheet open, and two open from a network drive. No modifcations to the sheet - just looking something up. Cut and copied data from a cell in Excel to Outlook email and mailed. No problem. Stayed in Outlook, and did a quick search against an Exchange public folder, and clicked on one of the query returns - an email sent a while back, which was a form post off our website. As I clicked on the post to open it (from the Outlook "find" window) and I suddenly saw my Excel sheets and the application itself close. Nothing else happend. The email came up fine. When I go back to Excel, it shows three "recovered" files, which were the three I had open.

I don't have any addins; and the only application I've installed recently, about 3weeks ago, is Ultramon, which helps in managing applications on different screens.

Any ideas? Anyone?