Excel error message
 

When I try running a macro, if I switch to a different program while it is
running I get the following error message:

The instruction at "0x3068013e" referenced memory at "0x0000000c". The
memory could not be "read".

Click on OK to Terminate the program
Click on CANCEL to debug the program

If I click OK the program closes, but then I can't get back into the
spreadsheet because it says I'm still in it. If I click CANCEL anotehr error
message comes up saying:

EXCEL.exe has generated errors and will be closed by Windows.
You will need to restart the program.

An error log is being created.

Then Excel closes and I can get back into the spreadsheet fine.

Any ideas what's causing these problems??