If you were trying to do something, and some random guy stopped by your house and said "don't do that," would you stop? Probably not, because you're balancing the chance of saving hundreds of dollars against the word of some stranger. "You don't know what you're doing" is not a reason to stop, because we attempt stuff we don't know how to do all the time. Human beings like to figure stuff out.

Of course, telling him WHY he shouldn't do this himself would be a perfectly reasonable response. It's not one I'd agree with in this case (installing an operating system, and even hard drive partitioning, are perfectly safe things to do as long as you look into the processes first), but I wouldn't view that kind of response as useless by any means. Explaining the risks is helpful, but just telling people what to do is not. I can't think of ANYONE, not even my girlfriend / family members, who would stop doing something they wanted to do just because I said so.

Also, for the record, I understand your frustration. I work as a computer programmer, and, while I don't deal with computer stupidity on a large scale like you do, I'm still "the tech guy" for my friends and extended family. I still maintain, however, that the solution to stupidity is education, not acceptance. "I'm dumb, I'll take it to someone else" vs "I'm dumb, I'll ask people and read the how-to articles they point me to."