If yours is a cannister with bumper strips, I think I know the answer, and devious doesn't begin to describe it! There are NO screws. The thing that's holding the case together is those long bumper strips along the sides. Open the front (where you put the bag), grasp the side rails and pull toward the front. Gently but firmly, if yours are as old and stuck as mine were. When you get them both off, you can gently pry the case apart. Watch for the wire connecting to the on/off switch. When you put them back together, make sure the motor et al slide into whatever grooves are there to guide them. Don't push the halves of the canister too far together, no yet allow them to drift too far apart...it's a delicate pain inthe tousch, but it works. You'll notice there are some prohibitive bumps when you start trying to push it in. Start it at a bit of an angle... You'll get it!
Chelsea, August 2004