Lotus Notes 6.5.5 crash

I have a user, whenever he open a email that contain a webpage, the hour glass will hang for some time after that a message pops up that says "NSD is running. Saving to file c:\lotus\notes\data ...." Lotus Notes will close by itself and when I relaunch it, an error says cannot access database.
I have to restart the PC and open lotus notes again.

any advise,

thanks