I'm not going to lie.
I've narrowed your computers speed down to one component: Hard Drive RPM. This is basically how fast your computer finds information and is the difference between when your computer came out of the factory and now.
For the quickest HD known to date, get a solid state disks. I would not suggest this, as they are expensive as heck. They are excellent, mind you, for a computer where all you do is word documents, photoshop, and internet. Boot up times can be around 10 seconds using these.
Run 'em with Linux though, not WinXP, and they are just incredibly fast. May not be the best for a teenager's main computer, as they like to put music and a ton of programs on their hard drive, and you won't be able to fit all this stuff on your hard drive if it's too slow. Used on Asus's EEE PC.
Another cheaper option is 15,000 RPM HD's. I have a 7200 RPM HD, so this cuts the load time of all your programs in half. It's probably the best option. A 73 GB HD costs $170.