For an immediate back slash, go to start>Search and search for On-screen keyboard, then click on the back slash.
Longer term, pop the key off and clean or blow out the area. Or just try blowing out the entire keyboard. Or, turn the keyboard upside down, bang it by hand a few times (on the bottom) and maybe gravity will take care of the problem.
I'm sorry. There is no key cap to remove. It has only a fwd slash, one the the main keyboard, and one on the number pad to right. Wow, I have two desktops in the room. Both have a dell keyboard. Neither have a backslash! Later I'll check my 3rd desktop way upstairs.
Edit: These aren't French Canadian keyboards, are they? If so, the backslash appears to be located where US keyboards have the "tilde" - to the left of the number "1" key in the top row below the ESC key.