Depending on the sort of hardware muscle required you might well be best going with specialized Industrial PC hardware. PC/104 embedded hardware, for example (
www.pc104.org is the trade group) or various other embedded style systems(google "industrial PC" to get yourself started) could do the job. Something from
http://www.hushtechnologies.net/start.html might also be useful, as those are designed to be totally fanless(The low power ones are even sealed, I think). Depending, also, on the nature of the OS and Apps in question, you may be able to get away with solid state memory in place of hard drives. You'll almost certainly be wanting to use good laptop drives at the very least, and solid state will give you much superior shock resistance. You shouldn't, of course, even think about letting read/write flogging occur on a flash drive.
More details about the kind of specs required would be handy, but I would suggest, off the cuff, something like the following:
Start with a hush mini-itx. The internal CDROM consumes one IDE spot, leaving three. Depending on the sort of shock expected you can either use a notebook hard drive(cheaper and easier) or up to three CF cards with adapters(slow; but bloody well indestructable). If you do this and are running windows, you'll either have to turn off the swapfile, or obtain a RAM disk for the swapfile. This gets pricey; but allows full solid state operation.
You could also go the industrial computer route:
http://embedded.kontron.com/ among others, looks like they know what they are doing. Drop them an email.
http://www.tactronics.com/ might also be worth a look.