This could be difficult. Security may get in the way of accomplishing this. Here's how it could work though--
1. First, set macro security to low so that it doesn't ask to run macros when a workbook is opened.
2. Next, set up a scheduled task (see the MS Scheduler for details). The command would look like 'c:Program FilesMicrosoft OfficeOffice 10Excel.exe "c:...MySpreadsheet.xls"
3. Create an auto_open() macro in the above spreadsheet with the tasks you want to do. You could have it minimize the application and close it when it's done. See Julian's Excel Solutions for ideas.
The biggest hang up is the the medium security dialog box that asks if you want to enable/disable the macro for the workbook. Running Excel in the low security setting is a little risky because if you open a spreadsheet with a virus, it will just run it.