D300s NEF files can only be opened by ACR 5.5 and above (Adobe Camera Raw, the raw converter in Photoshop). ACR 5.5 will only work with Photoshop CS4. ACR 6.x will only work with CS5.
Photoshop 7 is too old, it cannot even open DNG files.
So you can either upgrade your photoshop to CS4 or CS5. or,
Upgrade to CS, CS2 or CS3 and use VIewNX (free from nikon) to convert to DNG, then use adobe DNG converter to open in CS, CS2, CS3. or
Export to TIFF, then open with photoshop 7.