I'll assume you just want a fresh start:
- start Backtrack LiveCD
- use gparted (gparted at command line) and delete all partitions
- shutdown
- put windows cd in and install windows using the wizard
option #1:
- download ubuntu 10.10 (or whatever flavor you want)
- follow wizard
option #2
- download virtualbox
- install linux flavor
option #2 would be better if you are just learning linux
https://help.ubuntu.com/community/WindowsDualBoot


