When you do take it to them, make is absolutely clear to them that they must NOT wipe the drive or overwrite your design files.
Reinstalling Windows does not require this.
If your hard drive is SATA (most likely is) then you can get a USB drive enclosure to put it into, this can then be plugged into your MAC and accessed to backup your files.
Try watching this first.
It may give you some things to check.