There's way too little info in the original post to really answer this. In particular, is this machine going to be storing unique data (backups required), or is it just going to be used as a facility (i.e. readonly data access)?
Most commonly, of course, machine will be access in read/write mode and backups are required. In that case, I'd start with this:
1) Get a high quality UPS to run the machine
2) Run Raid 5
3) Do nightly external backups which are moved OFF SITE. (Either tapes, an on-line backup repository, or external hard drive that you remove from the premises). Don't forget that lightning strikes can fry EVERYTHING attached to the PC, powered at the time or not and that a fire or flood can destroy everything on the site. If you do incremental backups, be sure to do periodic full backups to prevent issues due to corruption of incremental backups.
4) Depending on what apps are required, preference would be for Linux in terms of system stability, but typically the apps are going to determine the system, not vice-versa.
5) Remove or lock from the user the CD-rom, the floppy, the USB and any other external ports that could be used to input data to the PC.
6) Install only what is needed on the PC and nothing else.
7) Stick with stock server-quality components that work well together, e.g. a field tested Intel motherboard with an Intel processor. Do NOT go with the latest and greatest; a half generation (or sometimes even full generation or more) behind is absolutely fine and preferable because those components will have proven themselves (or not) by that point. Get a server quality case with good ventilation. Don't even think about stuff like overclocking. If the apps to be run on the PC are not graphic-intensive, get a card that runs cooler and draws less current rather than the latest/greatest/hottest running card on the market. Use ECC memory.
That is only some of the basics. In terms of bullet-proofing the OS/apps...it's basically impossible. You cannot underestimate the computer stupidity of some users. But if the users only need to access a small, select set of apps, you can design a desktop that only allows them to access these apps and no others.
Kwad