Vista Drivers are compatible with WIndows 7. If devices do not enumerate correctly you have not installed the chipset drivers OR there is some other issue.
Doesnt seem to have made any made any difference. I cant see the modem when using Dell Diagnostics either, so I guess it must be faulty.
In your situation where you have installed drivers out of order, the chipset driver should be installed in Safe Mode. Then the NSS should be installed next.
If this doesn't work, OS re-install is necessary.