I always cook roast beef in the slow cooker - turns out beautifully.
What cut is it? (ie - inside round, cross rib, prime rib, blade, etc?)

Blade roasts make the nicest slow cooker roasts IMO, because I love a moist, falling apart flavourful roast. I usually put it in the slow cooker frozen about 4-5 hours before dinner. I season it with a little salt and pepper, a bit of garlic (powder or minced cloves - 1 or 2) and a beef boullion cube, crumbled over it. Then I add one chopped onion, and a diced tomato. (tomato is optional, but we love it) Throw a tablespoon or two or margarine on top, and then pour boiling water into the slow cooker. (About 3-4cm deep.) Put the lid on, and cook on high for 4-5 hours.