If you do a Repair Install, you basically over install the the old install, but you keep your installed files and data intact...
if you do a regular install, you basically install another version alongside... if something goes wrong you may loose your data...
it is generally adviced to backup any data before doing an install, it is always better to be safe than to be sorry later...
Ben
"If it works don't fix it! If it doesn't use a sledgehammer..."