The Task Manager Virtualization column corresponds to UAC file and registry virtualization, a compatibility feature. Please see the following article for a high-level overview of file and registry virtualization (second bullet point in the article):
http://blogs.msdn.com/uac/archive/2006/02/22/537129.aspx
For some details on when virtualization is enabled for a process, see the following comments:
http://blogs.msdn.com/uac/archive/2006/02/22/537129.aspx#549260