default program
If you are running the portable version of LO, refer this thread. If you are running the standard (installed) version, refer this thread. The Microsoft link in the answer in the second thread is probably what you are after.
EDIT: If you want to do this via a batch method for all required file types, you will need to make use of the assoc and ftype command prompt tools and create a script. Adapt the instructions provided in this mail list entry.