Problem partially resolved by uninstalling Grisoft AVG.
Will now need to ask if Grisoft are aware of the foregoing, and whether MS is able to shed any light on this.
However, I am not too hopeful of an explanation.
Running W98se, I accepted MS's suggestion to upgrade MSN Messenger from version 6.1 to 6.2. I also downloaded and installed critical updates from Windows Update, which included an update to Internet Explorer. My pc now freezes after booting up, and restoring to an earlier registry (scanreg /restore), will not rectify this. Add & Remove Programs shows both versions, MSGR 6.1 as well as 6.2, and each of them resist any attempt to uninstall. I am able to run in Safe Mode, and have run all spyware tools. Any suggestions please?