I get a lot of old PCs from friends' mothers.
For the outside, generally Colin works well. For really grimy and maltreated cases, petrol works well (the pure white stuff you get in electronics shops) but it's not paint safe. For paint finishes generally warm water and soap works perfectly fine. I prefer a diluted shampoo or hand soap as it's gentle on the plastic/paint.
I generally take the whole thing apart and treat each part separately. The metal cage responds well to a small shoe brush, some form of air delivery (blowing works fine) and some elbow grease. I may wash the inside or parts of it if it's got stained (don't ask, and you'll be told no lies).
I wash all the expansion cards and the motherboard. The CPU generally needs to be washed too. Nothing scientific, just a water + alcohol wash. After drying out totally a rub down with Acetone or rubbing alcohol gets all the residual stains out. It also destroys any stickers on the mobo/cards, so not always advisable. Generally the water works perfectly fine on its own, as long as it's not too hard. Hard water will leave mineral deposits. This is how PCBs are cleaned after soldering at the factory (+ some active flux removers), so it's perfectly safe as long as you dry them properly.
Fans are tough little buggers to clean, as are heatsinks. For Heatsinks I use a water jet between the fins (remove fans first). A toothbrush on fans works reasonably well, but the best is a good hard paintbrush. I've never been able to restore fans to a 'like new' look, FWIW.
Some parts of the cabinet may have rusted. A light coating of vaseline followed by some vigorous rubbing usually gets rid of most of it, and improves the appearance. Sanding and refinishing is an option, but it may be cheaper to just put a sticker on top if it's a visible area.
A blower/vacuum cleaner works really well. Just keep a distance, and don't blow it into the blades of a fan to see how fast it will spin. It's great fun though. For all loose dust, cobwebs, dead cockroaches and that piece of pizza that disappeared in 1984, there's nothing to beat a good vac.
As for static charge, generally static builds up on everything quickly (even brushes and cloth), specially in winter and when it's dry (Delhi). Be careful with anything you touch to the boards. I haven't managed to lose anything to static even though I'm not particularly careful, but in India we don't have carpeting (mostly) and it's a little more humid so it isn't such a big problem. As long as you're about a foot away from the board with the tip of the vac, you should be fine.
The Bheem Singh trick also works remarkably well for rigs with very little dust, which is a rolled-up kitchen cloth swung around with a lot of zest inside the cabinet. Though it can't get into crevices, it keeps visible surfaces clean by the air movement. It may tear the cloth in a couple of places, but hey, it's Bheem Singh, right?