The guy at the local PC hardware store told me it's better to use a small hand-held leaf blower to blow dust out of your PC case than a hand-held vacuum cleaner. Why is this? I understand that fans might be damaged if they "spin too fast", but surely this is a problem with both vacuuming and blowing dust away... I imagine both machines would generate a small amount of static to be discharged carefully, so I can't see how one would be "safer" than the other?
You can use both. Before ditching my desktop collection, I used to clean them using a six-gallon air compressor and a vacuum. Compressed air is much better at dislodging dust. As Nanne said, though, don't get your fans spinning too fast or you'll be generating current to be sent to your mobo. I usually use a finger to hold the fan in place while blasting it with air.
Anyway, unless you have a space away from where the computers normally live to clean their insides, you'll want to use a vacuum to suck up all the dust dislodged by the compressed air. Otherwise you're just going to end up with a bunch of dust in the air which the computers will pull right back into the chassis after you've powered them back on. I use short blasts of air to give the vacuum time to get the dust.
Without a vacuum, though, just use compressed air in a different, non-computer room.
Edit about the vacuum: You don't want to get it close enough to zap electronics; as others have mentioned those nozzles (plastic especially) can generate charge enough to zap things. You just want to capture the dust kicked up by the compressed air, not clean the components directly. Thanks to everyone who pointed that out.
A vacuum might be alright for slightly dusty computers but I use compressed air. Not those silly overpriced little cans, which do a poor job at the best of times, but from a proper air compressor. To protect the fans I hold the blades while blowing into or very near the fan.
Just one thing to be careful of when using compressed air, don't hold the nozzle too close to the computer. I keep it back about a foot or so. Get too close and you can unseat most components, including RAM and CPU.
For computers it doesn't matter too much; there is a danger that you can force dust into places it wouldn't otherwise go, but other than that, not a big deal. Fans will go bad on their own time, whether you like it or not. Static isn't much of an issue. Just leave it plugged in, and it'll ground out harmlessly.
NEVER EVER EVER USE BLOW ON A PRINTER. ALWAYS SUCK. ALWAYS Old school impact printers can handle dust, but they're about the only ones. Inkjets are more tolerant, but print heads are pretty fragile, and it can screw them up pretty badly. If you blow dust around in a laser printer, you're likely to push some up into the optics. If you do that, they basically have to be replaced and your print quality goes straight to hell.
Was he selling small hand-held leaf blowers?
I think this question is going to delve deeply into the realms of the subjective. Several points of physics come to mind as pros and cons for both sides, and a the net win might depend a lot on the individual vacuum or blower.
Personally, I think I'd go for the vacuum to clean out the case and any large deposits so they don't get blown around the room, then the blower to dislodge dust and get it out of all the internals thanks to it's much higher air volume and velocity. The second part I would still do with a vacuum handy to keep the work-space clean and filter it out of the air too.
As vacuuming doesn't remove all clogged dust, I'm fond of a small preassure tank with an airbrush nozzle that can deliver both narrow and wide air flow. One can blow 3-4 computers with a single refill.
To tackle the floating dust, I either use a table-top vacuum or electrostatic feather duster, depending on the shape and size of the case. When cleaning a server rack that can be opened from both ends, have another person hold the duster on the opposite end. Don't take the duster too close to the case to avoid static charges!
I mostly use a compressor and then blow it away with an airgun, this is the only way to get all dust loose (well, atleast at 10bars it does)...
When there are large dust gatherings a vacuum cleaner would also do...
As for the static... don't worry to much, just turn the machine and powersupply off BUT DON'T UNPLUG IT, this way the machine stays connected to the ground wire and is able to discharge if any static charges might occur, But honestly, the chance of a static charge forming so big it would actually damage components is minimal...
My preference is definitely blowing, as it's simply much more effective. However, it's messy and pointless if you're indoors. If you're unable to take the machine outside to do your blowing, you're better off using a vacuum cleaner. You can buy static-neutral vacuum cleaners with useful attachments that are specifically designed for this type of indoor work.
Generally speaking, you want to blow because it's a lot easier to concentrate blowing force than vacuuming force. Besides, the risk of ESD is higher with vacuuming.
The OP and others have mentioned using something like a leaf blower to clear dust from a computer. Actually, American vacuum cleaner manufacturer MetroVac makes an electric blower designed specifically for blasting dust out of computer equipment. It's essentially a high-power vacuum motor in a can, configured to blow rather than to suck. Here's a video of it in action.