If I delete my unused disk space with "eraser" will it delete my entire hard drive?

So, I've been planning on selling my laptop to a friend but I have really important business stuff on it. So I looked up a program called eraser that basically overwrites unused data so deleted files cant be recovered. After I opened it, it said I can delete all unused. My question is; If I do that, will everything be deleted? I want to keep my installs because my friend wants my adobe and stuff, so please answer me fast. Thank you very much.