the setup file will always be avilable from www.slysoft.com
some people like ot keep a prior version just in case, but in my opinion, there's no need to. slysoft releases versions that work.
also, the key doesn't have to remain on your hard drive, but i'd keep a copy on the hard drive, on a cd, on a floppy, wherever it will be safe. if you ever have to do a format or if you get a virus then you may need the key again and it's a big hassle for slysoft to issue you a new key.
along with the key, i'd keep the confirmation email backed up as well. if you ever DO need a new key then you'll need proof of purchase and this expedites that process.