Error During Install of Programs

I have a Vista Business edition installed and it's been added to a Windows
SBS 2003 domain. The primary user has UAC turned off. The primary user has
been added to the local administrator for his domain account. The primary
user is unable to install any programs. Once we login with our domain
administrator user or local administrator user, then we are able to install
programs. In addition, if we provide the user with domain admin rights at
the server he is able to install programs. As soon as you remove the domain
administrator rights on the user they are unable to install programs again.
We have also tried another user with the same results as above. We ran
Sysinternal's Accesschk on his hard drive and registry and all permissions
looks good. We receive the following error, "The folder path "My Documents"
contains an invalid charter whenever we try to install something without a
user with Domain Administrator rights. Please help!