I've had Dells exclusively for about 10 years now, and have been well-pleased. I've heard a few horror stories, but in proportion to the number of "satisfied" customers, the percentage seems really low.
The outsourcing of Tech Support is troubling, though. My first experience with it was extremely frustrating, my second much less so. So the jury's still out afaic.
But I will say this - as far as being ready out of the box, no tricks, no gimmicks, no surprises, no screwups, and staying ready to run, I've not seen or heard of any better PC. If your mother-in-law's not going to be engaging in any "risky" behavior computer-wise, I think she'll be well-served by a Dell.
Still, buy the extended service plan anyway. One accident and it pays for istself. And they WILL come to your house for free if you play super-dumb and/or scared shitless when they try to walk you through attempting to fix it over the phone first. Of course, if you like to learn on the fly, like I do, the fixing it by phone is pretty cool, But I know soem folks who aren't, who said "NO WAY", and they got in-house service w/no questions asked. GOOD service, too!
No such thing as perfect, to be sure, but I've still got no reason not to buy a Dell the next time out.