Why a Win 7 usb installer made using Boot Camp, is not working?
Windows 7 has to include Service Pack 1 (SP1).

Next, older Macs install Windows to use a BIOS boot method. Mac computers can not BIOS boot from flash drives. 

If you have a working optical (DVD) drive, then you should use the drive to install Windows. If your optical drive is broken or missing, the you can still install Windows, but the procedure is fairly complicated. See Installing Windows 7 or 10 Pro 64 bit without DVD or Flash Drive for the procedure.

Some really old 64 bit Macs can not install 64 bit Windows without first modifying the Windows ISO file to remove the EFI boot method. This only applies when using the optical drive.

Officially, your Mac can only run 32 bit version of Windows. This is also true for my 2007 iMac, but I was able to run 64 bit Windows 7 anyway. So if you have 64 bit Windows, then the only way to know would be to try installing.

The Boot Camp Support Software 4.0.4033 for your Mac can be found at this link. Newer versions of the Boot Camp Support Software (Windows Support Software) will not contain the correct drivers for your Mac.

If you have problems installing the Boot Camp Support Software 4.0.4033, then read the instructions given here.

Just so you know, older Macs can usually run Windows 10. The oldest Mac I have running Windows 10 is a 2007 iMac. If you want to try the current version of Windows 10, the you can download the ISO file from this site. You do not need a license to install, but to continue to use Windows 10 you will eventually have to buy a license.