Unless you delete a partition, or change the flags, you aren't going to screw it up. You'd have to purposely do that. EaseUS and gParted are pretty straight-forward.
For whatever reason I thought you were trying to resize your C partition. I've kind of re-visited this thread and what I'm getting out of this is you now have XP on C, and you have 2 other partitions that you haven't done anything with, and that you want to get that space to C so you have a lot of space on 1 partition? Is this correct? Apologies if it is not, I probably should read the thread in its entirety.
BUT, if that is what your end goal is (get 1 big C) then just use EaseUS from within Windows and right click on your I drive from within EaseUS Partition Master and select 'Merge'. Merge that into your D. Apply those actions. Then you should no longer have I, and probably an I directory on your D. Now do the same for D, right click on it, merge, merge it into C. Apply the actions. Problem solved.