Perhaps this is an issue that should be brought up with the makers of blender, perhaps they could help, if they don't know nobody knows.
But Blender starts fine? You just cannot import .nif-files with the nifscripts?
If Blender starts up, it recognises your Python (it's Python-based, after all). If you just cannot use the nifscript, your Blender probably isn't set up correctly - happened to me as well.
After starting up, click on the button in the lower left corner of the 3D view (the button with the grid on it), then select "User Preferences" in the fly-out menu that appears.
In the User Preferences menu now, select "File Paths" and in the second column (from left), the lowest entry specifies the path to your Python scripts. Enter the path where you installed your nifscripts (for me, that's "C:\Documents and Settings\*my username*\Application Data\Blender Foundation\Blender\.blender\scripts\").
Then open the fly-out menu in the bottom left corner again, then select "scripts window", then hit "Scripts", select "Update Menus".
Now it should recognize all scripts you installed for it. To make the changes permanent (i.e. show up on every start automatically), select the 3D view from the fly-out menu again (after all, you want this to be the first thing you see after starting Blender) and hit "Ctrl+U" on your keyboard and select the appearing option to save the current setting as default.
Cheers, LT.