Your not the first to have problems with installing Norton products. I assume that with NIS you have included NAV in which case Symantec themselves preach extreme caution before any installation. They emphasise that you must ensure that any other version of AV is cleanly removed otherwise there may be conflicts. I take it that F-assure was removed before you started!!!.
I had a similair experience and the only solution was to bring Windows up in "Safe Mode" uninstall Norton and used "System Restore" back to my last known working system.