Invalid page fault when starting

I get "Excel has caused an invalid page fault in module
rpcrt4.dll at 017f:7fdb02ae... etc." when trying to
start the program. The worksheet loads, then the error
comes on and after clicking ok Excel closes. After that I
can start Excel again, but I get the message that it can't
access any files, but it remains open. I have to reboot
the computer to get the original page fault message
again. However I have noticed it only gives me the error
message when Outlook in open at the time. It works ok if
Outlook is off, and I haven't tried other office programs.