1) Those who just use computers as they come out of box. These users will have Windows 7/Windows Vista and therefore are capable of running IE9
2) Those tech savvy people who insist on installing XP. They are smart enough to install and use some alternative browser (fx/chrome)
3) Those who are using that 10 year old computer and are using it as it came from the box. These people are the problem (in addition to corporate droids)
Most netbooks sold in the last few years have run Windows XP Home, and some continue to ship with it. Granted, many of the buyers probably fall under your type 2, but not all of them.