The Bethesda nif exporter and havok plugs both have a bug that causes them to replicate their menu entries until it starts to cause UI lag and crashing, I've fixed this for you. I sent the script fixes upstream to them but have never seen an update pushed so I'm putting it here so I don't have to give it out manually anymore.

See shortcut included in this package for more information about this problem.

INSTRUCTIONS
(yes you have to follow these steps, read them slowly, read them again more slowly, I may not answer questions or respond to posts)

to fix this , paste this line in the little text box at the bottom corner of the max window and press enter to open your menu file to gets its full name/path don't edit anything yet.

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edit (menuMan.getMenuFile())
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Open a text editor, navigate to that location you see in the titlebar for the opened max menu file. and open it in your external text editor.

Close 3dsmax, it will re-write that file from memory when it closes which is why we don't bother editing it in the maxscript window.

Now, taking care not to muck up the opening or closing tags in this xml file, delete all the records for havok/bethesda that are spammed in there 1000x, save the file.

Make sure you copied the fixed scripts into your 3dsmax/scripts folder and overwrite the old ones, if it doesn't ask you about overwriting them you somehow goofed up copy/pasting files, look harder, figure it out.

Now you should be good to open Max back up and get to work making some cool stuff.

Thanks Tobbe for pointing out the issue

-Cancerous