Newer OS versions aren't just marketing ploy, they also include major security features. This is very important if everything you do is in the browser. There are a world of browser exploits that can compromise your system, and the majority of them target Windows XP due to its security holes (some of which cannot be patched without moving to 7 or 8).

You can continue to make your stand against Microsoft's evil plans, but saving yourself $100 by not upgrading could end up costing you everything in your bank account when you end up with a rootkit that steals your account information.