Last March, I got my wife the Dell Mini 9 for her birthday and I have used it on rare occasion. It came with XP. I found the start up time to be slow and the performance to be sluggish. Functional for my wife for note taking, web surfing, and emails, though.
With the $200 special on the Vostro, my wife got me that for Christmas. It came with Ubantu, which I used for a few days. Really, I thought Ubantu was an improvement. The start-up time was faster and the applications ran about normal speed to me.
I followed the guide on here, got Snow Leopard, and used the USB install. Wow. What an improvement in performance. Start-up time is fast and applications are fast. I wish I could install this OS on my laptop.
If you are dissatisfied with XP, you might want to take a look at Snow Leopard.