If your getting a new Motherboard put in, then its no use having a BIOS password as the BIOS chip is on the MB...which is being replaced.
It would be best to back-up to CD, DVD, or other media, all of your important files. Most service centers have a policy that states they don't guarantee that your files wont be deleted!
Also, if they are installing a different version or type of MB to your original, then they will need admin access to your Operating System, to install any new drivers.
Baicly....if your worried about the safety of your files, back them up to some type of media....and if your worried about the privacy of your files, back them up then delete them from your PC.(Note: if they really want to read your files they can always use a data recovery program.
Most Service guys in those big companies can't be bothered snooping around your computer, as they have to account for the time they spend on jobs.