Sorry for delay but big issues with this. Using Avast clear in safe mode after repair resulted in same symptoms ie PC restarts soon after login screen appears or also if I have logged in to windows resulting in desktop appearing. Sometimes, system just freezes and only way out is to power down. System is XP Pro SP3 and no problems with Pro Antivirus. After 3 attempts to repair/clear/reinstal, gave up and downloaded Avast free with no problems. This auto restart after login screen suggests incompatibility with XP and performing chkdsk on boot up shows no problems. CCleaner also shows insignificant issues in registry clean. Sometimes message, “Embarrassed, AvastUI did not start” but trying to restart service from link fails. I am using I’net security on separate win 7/win 10 dual boot system with no problems. Win 10 updated to latest program with no issues but have delayed updating win 7 until this issue is resolved. Many thanks for your suggestion, I had forgotten about Avast Repair from control panel/uninstal programs