Personally I've done the whole hoovering before. I have both a camel hair attachment for my Rainbow and a cheezy little PC-specific hand-held vacuum. Both work alright, I guess. I've never tried putting the hose on the exhaust to blow the dust away, but that'd probably work better than sucking it up.
But some dust can be stubborn, and some dusty places hard to get a vacuum close to, so I've found that compressed dry air works a hell of a lot better IMHO. It really blasts the crap out of dust with a long narrow high-pressure tube. It's hard to beat that. (Especially for cleaning out PSUs, HSFs, and optical drives.)
<i>Good</i> ways to clean a PC out aside, I have to admit that sometimes when I'm lazy I just blow on stuff and/or use a paper towel to wipe things off with. :O
In any event though, if it's a <i>really</i> dusty PC and you're blowing the dust by whatever means, I'd suggest getting a dust mask like you'd find in a hardware store for wearing while you sand stuff and crap. It beats choking, gagging, and/or sneezing on dust if you get a bad whiff. **ROFL**
Another weird thing too is that sometimes I find little gnats dead in my PC when I clean out the dust. If you have fish they'll love the treat. (Insert debugging jokes here.)
<pre><font color=purple><i>Jesters do oft prove prophets.</i> -Regan in
King Lear (Act V, Scene iii) by William Shakespear</font color=purple></pre><p>@ 187K -> 200,000 miles or bust!