How about leaving the new hires on XP?
As Jordan has remarked, Win 7 is not going to function with IE6 or IE7 easily, or at all, and getting it to work would probably take much longer than either running the XP layer on Win 7 or leaving some users on XP.
HOWEVER, another thought has occurred to me. Do you really need to run the IE6 session on the Win 7 box? How about hosting it on Citrix or Terminal services if you have that sort of infrastructure in place already, or alternatively, how about having a few boxes running XP in a room somewhere, and allowing the users to use a remote session into the machine as and when they need the IE6 access.