Opening Instance Of Excel Through Asp

when i try to open an excel spreadsheet using the
following code, the process just hangs IF the spreadsheet
contains macros:

Set objXls = CreateObject("Excel.Application")
objXls.DisplayAlerts = False
objXls.Visible = False
objXls.Workbooks.Open my_excel_path '<-- hangs here

this worked on iis4, but since moving this to iis5 it
fails under the aformentioned circumstances. my guess is
that the macro security has something to do with it as a
dialog box will pop up if macro security is set to medium,
but i'm not sure this is really the problem. if it was,
what profile should change the macro security on the
server? when task manager is opened on the server to see
the hanging task, it shows SYSTEM as the process owner...

any ideas?