do you mean you have two partitons with windows on it !!
If you installed windows on the partition that had the old system .. that's mean it's very hard to recover it !
I need more info ..
how many hard disks you have ? where did you install the new system !
where was the old one !
if you have more than one primary partition you can swich the boot between them by marking the primary partition as "active"
Regards