Well,
I have never had any issues streaming .iso files over a wired network with XBMC, in fact, the way I am setup right now, is I have a HTPC in the living room, where I have all my .iso stored. That is connected through wireless G to my router. My xbox is wired to the router. This plays .iso just fine. The only time i had stuttering issues was when i tried to connect wirelessly with the xbox.
did you try to rerip to an .iso on the new hardrive and see if you can stream that new .iso? I don't know much about these things, but maybe your old iso's were corrupted in the transfer or something?
As a side note, what happens when you mount your old iso's on the XP PC? Can they play ok on the XP machine? Just in case you didn't know, you can download Slysofts/Elby Clone Drive for free for your XP box, and that way you can mount the iso on the PC. (Sorry if you all ready knew that)
-PTF