+1

Of course, even if you carry the whole desktop on in a rollaboard, I'd still reccommend taking the HDD ouf and carrying it separately in a padded case. The HDD is probebly the single most sensitive component of a desktop PC because of the moving parts and the precision needed for the heads to properly align with the platters. Modern HDDs are surprisingly robust, but it never hurts to be a little extra careful.

Can you at least get away with ONLY taking the box with you, and finding a monitor/keyboard/mouse at your destination? I'd be pretty nervous about carrying a monitor, and the kb/mouse are just extra weight.

If you're going to carry it on, you could also try a gamer's harness, or a full PC travel bag.

A gamer's harness is a harness made from nylon or polypropylene webbing, designed to wrap around a mid-tower case and give you a carry handle on top and a place to clip a shoulder strap. Most don't give any padding or protection, but they do make it a lot easier to transport the case.
http://www.amazon.com/GearGrip-Deskt.../dp/B00021UJ62

There are also some travel bags available for PCs:
http://www.frozencpu.com/cat/l1/g31/Travel_Cases.html

http://www.lansoft.ca/products.html

And lastly, crazy as it may sound - scour your company, neighborhood, and friends/family for a physically smaller, lighter case, perhaps a mini-tower, into which you could transfer your PC. It's the same computer, but if you can squeeze it into a smaller case for the duration of your trip, it will be easier to fit into overhead bins and lug through airports. The transfer is mostly just turning screws and plugging in cables, and can typically be done in less than an hour.