This may sound like a silly question - so please don't be offended - but why are you connecting your iPad to your computer?
I say that because, as several Forum members have suggested, it's hardly worth connecting your iPad to your PC simply to 'back up' the iPad.
For one thing, backing up can take a long time. For another, since iTunes and the App Store keep a record of all your purchases, even if you 'lost' everything and had to completely reset your iPad, you could upload again all your apps and iTunes purchases. So all you really need to back up are any documents, presentations etc that you might be working on. Photos - well, I guess you've got those on your PC anyway.
Backing up documents etc can be done either directly to your PC over WiFit using, say, GoodReader or another third-party application or via a 'cloud server', such as Apple's MobileMe iDisk.
OK - I know this doesn't really answer your question (well, actually it doesn't answer it at all!) and if you really want to sync contacts and appointments all the time between your PC and your iPad it's definitely not a solution, but I just thought I'd mention it.
Going back to your original problem, as the previous poster suggested, maybe trying a different USB port (if you have one), checking you have the latest XP Service Pack update is the next step.
Keep in touch and let us know how you get on.
Tim
Scotland