SW would not let me update 17 to 18. Now I have both on my computer. Do I just go to my control panel and uninstall 17 or would that cause 18 to malfunction?
Unless you want both versions installed you should always try to do a complete uninstall before reinstalling a new version. Javelin has a really good guide to follow here : http://solution.javelin-tech.com/l/2012/2012-11-25/xycvy/2012/124608/solidworks_uninstall.pdf
So with that said should I delete these old files from my C drive too?
If you're not running multiple versions then yes, get rid of all that baggage.
Just make sure you have a safe copy of all your data, templates, config files etc. before you delete anything.
Retrieving data ...