OK, you might want to try this. Buy a Wireless Router (802.11n routers can be gotten for 20 bucks, .11g for $15, and .11b for $10), and then set this up to use the XP machine for internet. so

Tablet < -- WiFi -->Router<------->Windows PC<---dial-up--->

I think there are tutorials on how to do this, because it is not tablet specific.

Edit: About the price... I'm talking about a good sale, not normal waltz into a store price. For example my router died, and my dad got a nice deal (no rebate or nothing!) on a new draft .11n router when they were new for under $30. So you have to look around, or risk getting ripped off.