Dell Tech Support

I have a four year old Dell XPS 420 desktop computer an approximately one year old Dell Laser 3115 printer.  About a year ago I started receiving an error message that said "download driver to unknown device" and at the same time I was having intermittent printer problems.  After several rounds with remote support, my problem was not solved and I ended up buying this top of the line, expensive printer.  The problems continued.  Now here we are about a year later and the scanner doesn't work on my expensive printer, I've had three motherboards and two power supplies installed and still my computer doesn't function properly.  I decided I would try another time with Dell and spoke to one rep for about an hour and a half.  He said he had to go and would call me back but didn't until the next day.  I wasn't home when he called and couldn't talk.  He called once again and once again I wasn't there and long story short, my problems have never been resolved.  Last night I got an error message saying something like there was a problem with one of the USB ports on my computer and the one identified was "unknown device."  Sure chilled my bones!  Anyway I emailed and then called tech support.  Three times I was placed on hold by the Dell rep answering my call, the last time for more than an hour with nobody ever coming back to the phone and finally I hung up.  I do have an extended warranty on the computer.  Also it is worth mentioning that I am a 70 year old woman with very limited expertise in computer technology.  I am at my wit's end.  My original problem seems to still be there and now there additional problems as well.  I purchased the extended warranty for (I think) $200 and then purchased a $700 printer and still have limited function on my computer and the scanner on my fancy printer doesn't work and nobody seems to want to help me.  Where do I got from here?  Help!