VERY Bad PC environment

Start by setting up the PCs so that you have a positive air flow into the case. You want the intake fan to have a minimum of 10% more airflow than all outlet fans combined.
(You may need to block some of the air vents on the case to accomplish this.)
Allow for the loss of airflow with a filter over the intake fan.

Then, install a high efficiency filter over the air intake. You may need to service the filters on a weekly basis, but it sure beets opening the case.

If this is not enough,build a case for the PC.
Set up one or both sides of the case with a mesh grill in front of a 1" or 2" furnace filter.
You can use a foam insert to seal around the cable inlet.
Make a self closing door for drive access.

While I have not seen them lately, there used to be some completely sealed keyboards with capacitive keys in place of moveable keys that were designed for harsh environments.
Check the tool supply and industrial computer catalogs for these.

HTH
Chas

How about buying some computer enclosures? There's an upfront cost but they should pay for themselves in terms of maintenance costs.
Here's an example:

www.itsenclosures.com/product_enclosures.asp?cid=1

If you're replacing them that often, you should easily be able to justify purchasing the "industrially hardened" pc's. Our industry, process control, has some very harch environs, & the industrial pc's hold up well.

Check out our web site at http://www.graymattersystems.com/gms_industrials.htm

for some ideas