How to clean hard drive for a HP a1720n desk top computer?

I qualify for the deal for students to buy Windows 7. Although, XP users must perform a clean install, as opposed to Visa users, who can perform an upgrade.

It seems that this deal is only for upgrade versions. Is this true? Can XP users use the student deal to install 7?

How do you completely clean your computer out like the first day you got it. I mean delete everything.

my PC is full of dust and I think bcz of that it is so noisy while running on. how can I clean my PC and take away from this noisy sound?