Since the tests are failing and you are having problems starting Windows, I feel comfortable saying that you are going to need to replace your Hard Drive. A drive failure can occur suddenly, so make sure that you back up all the important information while you can still use the computer. I would also highly recommend that you start looking at options to replace the drive and install Windows.
If you are under warranty, Dell should be able to help you resolve the issue, and if not they should at least have some (relatively) inexpensive options.
If you don't go with whatever options Dell has available, then you will probably want to find a local technician to do the work for you. Unless you are comfortable replacing the drive yourself of course.
Unfortunately, there is no defined cost for PC repair, but back when I worked a repair desk (2006) the typical charge to replace a drive and install windows would have been about $200 (or about £125). Of course this was several years ago, and each repair shop is different.
Please let me know if you have any other questions.
Thanks,
Kris