I’m using Windows 7 - it happens with both switching user and logoff - log on to another user and then log off and log back on to the original user. It sometimes happens at reboot and sometimes on user switching and sometimes with just a logoff-relogon to the same user. I don’t know how to switch off user swtiching on 7 - can it be done? I have dropped a couple of screenshots on sendspace for you. Asd you can see twitter and the Avast gadgets move to the right overlaying the ones that are their already.

Normal Desktop arrangement

Scrambled gadget positions