A common problem if the wheels haven't been shifted for a while.....get a piece of wood and a large hammer - put the wood against the tyre inner side and give it a good whack.....turn the wheel 180 degrees and whack it again...eventually it will come free. resist the temptation to hit the rim directly though - especially if its an alloy one!
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.