I've got a hoover self propelled wind tunnel vacuum cleaner. The belt won't stay on.?

Other than to recommend that you take it to a vac repair shop to have them properly install the belts and make sure they're right, there's nothing much else to say. I've been doing vac repair for over 35 years, and have had customers bring these machines in with new belts that they've installed, completely wrong. Once properly installed they worked fine. There are a couple of places that must be checked to make sure the belt is where it's supposed to be and the shop will know where that is. If they then have problems with the belts coming off they can determine why.