Can I set the receiver on it's side

It's probably OK, there have been posts from a number of people that have mounted the DVRs vertically, but that's usually with the front facing outwards. The big issue is cooling - it needs to be in the open, not inside a cabinet, at the very least you need to make sure that none of the cooling vents are obstructed (probably nothing within at least six inches from any vent).  You can't just push it sideways against a wall and expect it to operate properly. And if you do it, check the internal temperature regularly. My HR24 runs at around 110.