My Ubuntu used to start really slowly, even more slowly than my former XP. It was not really a problem except on the mornings when I wanted to surf while eating my breakfast.
Enjoy the guide here
First I installed a Bootchart. It's not necessary, but it will tell you exactly how much time your computer needs to boot. If you boot your computer before any changes and after the changes, it tells you how much faster it's actually starting. So it's recommended.