Moved this here from the Experts forum *cough* and modifed the title a little.
Im hoping this will act as a proverbial "pigs head on a stake" and help educate others on the pitfalls that can occur when you don't carefully read instructions before following a potentially destructive process that you don't fully understand.
OP, when you ran the install.sh script from the BackTrack desktop, you installed BackTrack to your systems hard drive and overwrote your Windows install. You probably should have paid better attention to that "Prepare Disk Space" screen in the installer.
Hopefully you didnt have anything important stored on the hard drive. If you didn't, just reinstall Windows. If you did, consult a data recovery specialist; OR you can attempt to recover the data yourself using one of the many freely or commerically available file recovery tools (although based on your efforts so far I wouldn't necessarily recommend this last option).