There is a knowledge base article (number 612594) that talks about the problem that you are seeing:
System Information for Vista 64-bit computers, reports as SYSTEM and not user Name
Environment McAfee ePolicy Orchestrator 3.6.x McAfee Common Management Agent 3.6.0 Microsoft Windows Vista (64-bit)
At the ePO Console under the computers System Information section the User Name property is reported as SYSTEM
This issue is only seen on managed Microsoft Vista 64-bit computers
Change or Cause
On Windows Vista computers the location in the registry where the User Name is stored is restricted. An alternative method of obtaining the data from the Microsoft Vista 64-bit computer results only in obtaining SYSTEM and not the user name.
At this point in time the McAfee Common Management Agent 3.6.0 is unable to obtain from the Windows Vista 64-bit computer the User Name until a new or modified Windows Vista API (Application Programming Interface) is available.
This issue is documented under Known Issues (item 3) in the McAfee Common Management Agent 3.6.0 Release Notes, see 651488.
The McAfee Common Management Agent 3.6.0 Release Notes, see 651488 have the following info under the "known issues" section:
For managed Windows XP 64-bit and Windows Vista systems, the system information User Name property is not reported in ePolicy Orchestrator. For these operating systems, the User Name displays "SYSTEM."