The Madshrimps could not be more right with this one. The casters they use are definitely one solution. I just used a couple pieces of 2" X 4" underneath my HTPC, though. Anything will work, & that front intake on the bottom of your PC will thank you for it...

"Most homes in the U.S. have carpeted floor; we all know too well most ATX tower cases anchored or sunken into the carpet with those tiny rubber pads called feet. Are you sick and tired of dragging your PC from under the desk out? How about letting in more outside air? $4 to get your rig higher on the rug in 5 minutes and get more air from your front panel for $1 in about 30 minutes."