The worst thing in my experience (besides f*cking up the person's computer), is having to tell them:

"You cannot do this, you need a new computer"

People hate hearing that. They assume they can slap that Radeon 9800 in their P200 and everything will be fine. "But it has MMX! It's a good computer!"

My roommate last year had a P120 or something like that. It was in such bad shape. Eventually I had to tell him he couldn't really do anything with it anymore, and that he could 2 generations behind current and still come off with 4 times the PC. He didn't understand a word I said.