Excel won't shut down

I am running Excel 2003 on Win XP and whenever I close an Excel workbook EXCEL.EXE will remain in my Processes list. If I wait for a while (usually somewhere in the neighborhood of a few minutes) it will go away. Otherwise, I have to manually kill the process. I am also unable to open any other Excel file until the original EXCEL.EXE process is killed. Any idea what could be causing this? This happens even when I open blank Excel workbooks, so I know there's not a macro constantly gumming up the works.