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"

You know, this doesn't bother me at all. I love telling people this because it effectivly ends my involvment. Sure they try to get me in on the purchase, but I just tell them to go down to Best Buy and pick up whatever looks good. The truth is that when a new computer is needed it's better for everyone. And PCs are getting so cheap these days, the last time I "upgraded" I just bought one off the shelf (and popped in a great video card, of course), which is the first time I've ever done that.