How to allocate office com interoperability in run time ? ? ?

Hi,

I need to control some office excel document in run time.

I have 2 problems

1. I don't know witch office version i have on current machine ( can be office 2003 or office 2007 )

2. I need to allocate the pointer to the office in run time - so i cant do in from visual studio by add reference in compile time.

How do i allocate the this pointer in run time and set the right pointer to the right office version  ? ?

Thanks for any help.