How do I save my iPhone backup once my computer is wiped?

I'm wanting to wipe my computer clean to start it all fresh due to not having enough disk space for anything.

I'm scared if I do that, that i'll erase everything on my iPhone (contacts, photos etc) once I plug it back in after everything has been installed and everything is new.

I have put most of my music and photos on a hard-drive so worst comes to worse I can put those back on manually. But i'm more worried about my apps and contacts.

How can I save these once the computer is wiped and I start with a new iTunes?