Machines that have been left for a long period of time in an outbuilding, shed or attic can get in some terrible states, though having said that in my last job as a field engineer I encountered some running kit that looked like it had been stored in a duststorm while it was still plugged in :) Some people are apprehensive when it comes to cleaning old machines because they think hot water and/or detergent will harm the circuit boards, others think nothing of getting a machine out of storage and giving it a rinse with a power hose.
Just think though of the process a circuit board went through as it was being produced in the fabrication plant. Back then anyway, I think modern construction processes are similar. For a start you had a wave soldering machine which is basically a vat of molten solder heated at over 400 degrees centigrade that the boards skim over the top of. They've already been treated so that the solder will only stick to areas it's allowed to which means the components are then soldered in tight.
Then the boards go in the industrial equivalent of a dishwasher and are cleaned with chemicals that remove excess flux (part of the soldering process) and generally clean the boards. Again heat is involved. This means that sticking an old circuit board or keyboard in the shower and giving it a scrub with mild detergent and a toothbrush won't harm it one bit. There are caveats of course, one of which is I wouldn't do this to monitors due to the massive stored voltages involved.
There's 2 schools of thought regarding power supplies. One side says it's OK and the other side says it isn't :) Personally I wouldn't, again because of the voltages inside......for boards themselves if there are capacitors involved then make sure they've had ample time to discharge. Also remove any batteries present and remember that the washing will probably remove any paper labels or stickers so they either have to be removed first (after noting their location) or treated very carefully during the wash.
If you're going to use a dishwasher instead, all the above things apply along with making sure you use little or no detergent and DON'T use the drying cycle, particularly if plastics are involved. They'll melt!
Drying should be done in a warm non-humid environment and should take 3 days or longer for boards and keyboards, casings and plastics don't need as much time as that. People in warmer climates than here in the UK sometimes dry machines outside because you get the added advantage of moving air to speed up the drying.
Finally, make sure the wife doesn't mind you messing up the shower and/or bath :o)
This machine was one of 2 PETs I recently rescued from a shed and boy did they look like it. Neither works so I figured the best thing to do first was to get the scrubber and toothbrush onto 'em so I could at least see what I was doing whilst troubleshooting. They also hadn't been powered up for 2 weeks. This is the first round of pictures; the rest will be posted probably next week when they've had ample time to dry out.
*Update* They're now back together as you can see in the last few pictures - hell of a difference between the 2 sets of pix don't you think? :) Both machines are still broken however.....