"should i use && oprator??? "
You can't replace the "||" with and "&&" - otherwise teh test will always fail as teh KeyChar value cannot both be Key.Back and Key.Space at the same time - for the same reason it can't be 'X' and '7' at the same time!
Put a breakpoint at the start of each handler, and follow it through - see what happens when you press the space bar and try to work it out from there.
BTW: Don't write it like this:
if (char.IsNumber(e.KeyChar) == false)
INstead, use this:
It's a lot more compact and more obvious once you get used to it!