Is this what you did?:
Try cd'ing from the command line into the folder with DiabloMiner in it. So:
1.) Download Diablo Miner
2.) Extract to folder in "Downloads"
3.) Start>cmd>Enter>Command Prompt Window Comes up
4.) Type "cd " (with the space) then drag the DiabloMiner folder over the Command Prompt. Press enter.
5.) Type in your
DiabloMiner-Windows.exe -u firstname.lastname@example.org -p password -o pit.deepbit.net -r 8332 -v 2 -w 128 AGGRESSION = 13
6.) Press enter
Did that work/make sense? If not I'd be happy to make a video tutorial of it...