let me put it to you like this...if you are anything but a newbie, you should reformat/reinstall (if you have a copy of windows ).
if you are a newbie, you should also do a reformat/reinstall. a. you could use the practice and perhaps graduate to a non-newbie, b. your computer will be MUCH more efficient with a fresh install of windows, and c. if you screw up, you can just put in the dell recovery disk and get back to where you started.
also, i would leave the 47mb diagnostic partition, just because it is easier to work with dell if something happened. if not, you can always boot to the diagnostics from the driver cd.