Firefox & IE using 100% cpu core

Windows XP Pro SP3 all updates AMD Athlon 64x2 4GB RAM. NIS 2014. About 3 weeks ago both Firefox 32 and IE 8 started to use a whole cpu core on startup never succeeded in rendering a page and became unresponsive.
This did not happen when booted in safe mode - no networking.
Did a System Restore to a date 1 week earlier - problem solved so I thought. Went on holiday for a week - 2 days later problem returned.
Used msconfig.exe to set Win only load minimum but problem still there. In desperation uninstalled NIS. Both browsers now worked fine. Took opportunity to instal Chrome. Reinstalled NIS - Firefox and IE no longer work but Chrome does.
My conclusion - problem caused by NIS. How to solve it?