He is most definitely right. If you go through your computer deleting things all willy-nilly, it will just cause problems. if you think you are getting low on space, there are several proper ways to reclaim space:
1) double click "My Computer", find the C: icon, right click on it, and go to properties. on the general tab right under the pie chart is a button called disk cleanup. this should do a lot for you.
2) open Internet Explorer, at the top, click Tools, internet options, and halfway down are the buttons delete files, and delete cookies. this will clear some space, but will affect your internet browsing, as this is where all your settings for the websites you have visited are located.
3) go to control panel, add/remove programs, and remove programs you are absolutely sure you will not use. removing by deleting folders is bad practice, and causes your registry to go all haywire on you, among other things. by the way, your registry is the place where your computer keeps track of what programs you have, your settings for each program, and a few other things.
hope this gets you started, and have fun.
