Can I use a trainer with a mountain bike?
I want to the same bike off road at weekends so I don't really want to change the setup of the bike.
You can use a trainer on a mountain bike, but a big issue that you will run into if you don't want to change set-up between the trainer and your rides on the weekend is that running knobbly tyres on a trainer is just plain noisy. If you are determined to run the same tyre on the trainer as off-road, I'd recommend looking into the tyres that have a raised, slick low rolling resistance portion in the middle, with knobs on the sides to help with cornering.
Another option which requires only a little bit of set up is get a wheel for use only with the trainer. If you hunt around buy & sell forums, you should be able to find a cheap wheel and cassette and slick tires only cost $20-30. When you want to use your trainer, just switch wheels.
You can, but it depends on whether the particular trainer will accommodate a 26" wheel. I have two trainers, one that will work on a mountain bike (found at a garage sale), and one that won't quite adjust close enough (so I use that one for a road bike only). Best check the specs closely before buying a trainer.
If you look at the various turbo trainers that are available, they should have some indication of what size of wheel they can accommodate in terms of the diameter. If you've got 26" wheels on your mountain bike, then that's the size the it'll need to be able to cope with.
Mine just comes with a special Quick Release skewer to use with the trainer (you swap it back to the normal skewer when you want to use your bike as normal).