I've had a similar problem, this might be the solution. The floppies that you are replacing with, are they all old ones or generally new? With XP it seems that it can't use old floppy drives (learned this from experience), try buying a new floppy from compUSA, bestbuy, etc. If it doesn't work then just return it but I'm pretty sure that's the problem.
BTW: this is my first post, what's up everyone.