If you do take it and store it in the hold, I'd suggest removing the harddrive. You don't want that bashed around by other suitcases.
Taking it as carry-on - as long as it fits in their luggage dimensions (the airline you're using will have this on their website) and is under their weight restrictions (you sure it weighs less than 8kg?), then there's no reason you shouldn't be able to. People take laptops all the time, and yours is just a bigger, bulkier version ;)
However, what I'd pay to see is the looks on peoples' faces at the xray screening checkpoint, where they tell you to remove the laptops from your bag. Naturally, I'd expect you'd probably have to take your desktop out of the bag as well, and it'd be fantastic to watch :)
A quick searched showed me that another guy has done this before, with similar reactions, and from the sounds of it he had no problems at all.