if the partition is large enough just boot Windows 7 DVD and do a clean install formatting the partiton to NTFS (again).
No need to use Boot Camp Assistant which removes and creates. You may use BCA in Lion 10.7.x though to download the support software you will need to install after Windows 7 is up and running.
If your imac is that old you may not be able to use 64-bit edition.create a Windows support software (drivers) CD or USB storage media
The Boot Camp Assistant can burn Boot Camp software (drivers) to a DVD or copy it to a USB storage device, such as a flash drive or hard drive. These are the only media you can use to install Boot Camp software.
- Installation Guide Instructions for all features and settings.
- Boot Camp 4.0 FAQ Get answers to commonly asked Boot Camp questions.
- Windows 7 FAQ Answers to commonly asked Windows 7 questions.
Macs that work with 64-bit editions of Microsoft Windows Vista and Windows 7
Some computers require Microsoft Windows 7
Option = Mac boot device manager to select where to boot from
x = attempt boot Mac OS X if possible