If you speak to people like that, I'm not surprised that they hung up. Unfortunately, your condition does not excuse you, it does mean that you must be very careful how you deal withy people.
First: Write down your problem, then look at what you have written and rewrite it. If necessary, a couple of times.
Then re-ring Dell and speak very calmly with them.
Tell them what you have written down and listen to their reply.
Ask them if they will give you time to write their reply down. Then write down their suggestions, so that you can reread them later. They may offer to eMail their suggestions and if you cant get your eMails ask them to send it to a friend.
Say thank you and ask if you can make a return call, if you get into difficulty.
p.s. I am an Asperger and have the God Given Gift of saying inappropriate things. So, perhaps, I have an idea of what you are feeling.