Just don't give them your hard drive. Have them take it out. If they want to test boot the PC's they should have extra Hard drives there to test it with. And when you go to pick it up have them slip to new Hard drive in. Simple as that.
AMD Athlon 64 3200+ Venice Core, MSI K8N Neo-4, 2x 512 Corsair Dual Channel, Nvidia Geforce fx 6600 256MB, 160GB HardDrive
Windows Vista Business Edition
"There are 10 types of people in the world. Those who know binary and those who don't."