You put your OS disc in, make the CD/DVD drive the first boot device in the BIOS, restart, press any key when prompted, use the R option to repair your system volume.
As long as you just use the R (repair) option, it won't erase any data.
The system repair is only necessary if it doesn't boot off your hard drive though. When you get your new hardware, make sure your hard drive is the 1st boot device in the BIOS, then restart. If it sits there at the black BIOS screen when you restart, then you will need to do the system repair.