My computer's insides are decimated, need a clean slate.?

I have windows vista. my sister, although she has her own laptop, feels the need to fill my hard drive with unnecessary, untrustworthy 3rd party software for media downloads, and fills my internet browsers with an insane amount of obscene tool bars and garbage. My computers speed has suffered for it, and i want a fresh start, but i don't have a system restore disk. Do I have any other options for resetting my computer to factory settings? Will installing a new OS give me the option to clear all of my personal files and settings?