Logon Credentials

I have installed Vista Ultimate on a laptop for a network test machine. The
laptop is part of a W2K3 Domain and I am a member of the domain admin group.
My problem is this: Whenever I install a new application or hardware that
requires a reboot, when I login on the machine, I am assigned a new user
profile and loose information in my previous profile. i.e. I am logged in as
username>@mydomain.com and my profile is held in
C:\Users\<username>.mydomain. When then reboot is finsihed and I logon
again, I am recognized as a new user and now my info is stored in C:\Users\
<username>.mydomain.000. ( I am now up to ...001 and also have a
"<username>~1~myd" directory. (The names have been changed to protect the
guilty). So, What goes? anyone got a clue?