Running net user from a dos prompt lists a user SUPPORT_388945a0. What
is its function ?

According to the description in user manager CN=Microsoft Corporation,L=Redmond,S=Washington,C=US This is a vendor's account for the Help and Support Service

I would imagine it is to give BillG access to you computer. I disabled it on mine some while ago.

I belive it is for when you request remote desktop assistance from Microsoft. If you don't require it, just delete it.