BleepingComputer.com: Windows XP Fast User Switching

My wife and I have been using fast user switching on out computer for years with no problem. Beginning a few months ago, I started Win XP Prof one day I get to the welcome screen with no user names displayed. I then press Alt+Ctrl+Del twice and type in the username and then am able to login to Windows with an account of owner. When I go to user accounts in Control Panel I get the error message cells.item(...) is null or not an object. When I go to computer administration and then local users and groups there are no users listed under users but the groups are listed. I have done system point restores and imports of exports of the registry I take with export. I have tried everything under safe mode and it makes no difference. This followed a Microsoft update the night before as I left the computer on. I tried to do a restore point but all my restore points were corrupt. I was working with a fellow from Microsoft and he had me do all kinds of things but to no avail. He seems to think a DLL file is corrupt but which one? Finally in desperation he sent me a new OS XP Prof SP3 disk and I installed it and it wiped my hard drive. Luckily I took an Acronis Image backup first so I easily restored my system. This has been going on for months,, I need help. Thanks - Johnny