As cheap as used computers are, I find it hard to believe that there is a market for computer rentals.
Anyhow:
Hardware, Hardware, Hardware!!!
Get a set of IDENTICAL PCs.
You need to walk a fine line between too much and not enough horsepower.
Stock up on CHEAP mice and keyboards.
Make the cost for a damaged mouse or keyboard reasonable to reduce confrontation with customers.
Work with your Microsoft rep, and get a corporate license so you don't need to worry about activation.
Create a disk image and ALWAYS wipe the drive and restore the image when a PC comes back in.
Install tamper proof seals on all removable parts.
Replace case screws with security style screws.
Use group policy to lock clients out of key folders and services.
Choose a diagnostic utility that runs from a bootable CD or thumb drive. Run the utility and printout the results at check out and check in.
Chas