IE4 Corruption

On several occasions, I have seen the IE4 browser on NT-based computers become corrupted for no obvious reason – no system changes having been made, etc. The only solutions I have found are (a) completely rebuild the user profile and laboriously recreate Outlook and printer connections along with desktop settings, or preferably (b) upgrade to IE5 which “fixes” the corruption. However, I want to know the root cause as even with IE5 the browser can become unusable but at least this version offers a repair option. Any suggestions? – all affected PCs are running up-to-date A/V software and are able to browse intranet / Internet resources with Netscape Navigator if installed when IE corruption has occurred.