Local Disk Problem....................?

So I happen to just got some computer repairs and the repairman added new hard disk in my computer. My old disk is still on my computer however he didn't remove the old files there(backup of my old files) and it's still there but I want to make a fresh start, my question is will it be safe if I reformat/cleanup my old disk? there's a(old) windows files there and I don't wanna take a risk in uninstalling it and getting errors again.