Got a kindle that won;t turn off but that is still in a cupboard somewhere.
Have a look at
www.ifixit.com - lots of advice on how to repair stuff.
As for the in-store service, I sympathise with the female assistant - we've all been there. Its the first time you've been on that section, or the customer needs some help now, so your doing your best even though you've barely set eyes on the kit before yourself, or the bloke who normally does that stuff is away or at lunch, or its the new version which you know nothing about (I'm a part-timer, so that really can happen). None of us are perfect.
And, as people are pointing out, proper sat navs are becoming less popular as apps on smart phones grow, so you seldom get asked about them, which means that its not something most people on the floor really know that much about (we used to stock a Pioneer TV which we all hated, because it was so specialised that even turning it on was a nightmare). Sometimes all you can do is read the label, and try to make sense of it to the customer (which is sometimes all they want). On the other hand, the computer guy was totally unhelpful - if I don't know something (which I'm perfectly willing to fess up to) , I'll find out or find someone who does.
Looking at what some people think about PC World on the 'New Computer Advice' thread, your review could have been worse!