The best option is to get a new hard drive. You can find one for $100.
Then just connect your old drive to the PC with a USB to SATA adapter and copy any files you need.
With the extra drive there is no risk to your data from the install as long as you DON’T CONNECT THE OLD DRIVE DURING THE INSTALL PROCESS, since you could conceivably choose the wrong install disk. If it’s not plugged in then you can’t choose it