Browser crash within case while entering Time

I have a few employees that have problem entering their time within a case.  Not sure if this is something that should go in the "time tracking" or "support" area, so I'm starting here. 
This problem occurs only to a few employees, which is why it's much harder to figure out where the problem lies.  The browser in question is IE v.6; when the employees are within a case, after entering all necessary case notes, then when they click on "Time" tab, their computer locks up and they need to reboot to get back to work.  I've tried having them delete cookies, history, temporary files, then we also went into NS/Home/Set Preferences and changed the "Maximum Entries in Dropdowns" to 25, quick search and popup search uses keywords both checked, and their system still