1) Check with the repair facility to see if they actually need your hard drive for the repair. If they don't, then remove the hard drive and keep it. When repair is complete, re-install the physical hard drive.

2) If the repair facility says they DO need the hard drive for the repair, then copy all important data files to removable media (an external hard drive, a flash drive, or some such). When (and only when) you're sure you've copied everything, use a program like BCWipe to securely wipe the remaining free space on your hard drive. Programs like BCWipe write random data onto the (now) empty space on your hard drive, up to seven passes, to thoroughly destroy the ability to discover or reconstruct what was once there.

3) Once your 'puter is repaired, get some decent backup software, set a schedule, and back up your important data regularly so this never happens again!