I had problems with the N900 (very short battery life and other issues) that I thought that reloading the maemo5 would help and had to find how I could reload it.

Found an easy way using Nokia's PC Suite and using the Update feature. I could not use the Update feature that was on the phone as it was already updated.
Using the PC Suite I was able to reload the OS and restore the phone to the original setting. I was able to fix the battery issue that way.