I am looking for the most affordable Rugged Tablet PC on the market
that has an integrated prox card reader.

Specs need to include:
  Windows 2000/XP Compatible
  Rugged or Semi-Rugged to handle dust environment
  WiFi enabled (Wireless Networking capability)
  Touch Screen (using finger)
  Integrated Proximity/RFID Card Reader

This rugged pc is simply to be used to develop our custom timeclock
solution for manufacturing plants.  Users will carry a prox card and
swipe it at the computer, which will clock them in using our custom
written software.

You barely need a computer for that.

It is little more than data capture
- the thing needs a brain the size of a pea, rather than an expensive XP machine