Vista - Taskbar focus in VPC

When using Action/full screen, I have a lot of difficulty getting the guest
machine's (Vista Ultimate) task bar to cover the host machine's (XP Pro)
taskbar. When I click on Action/full screen, the guest machine's task bar
should cover the XP task bar automatically. One out of ten times it will do
this correctly. The rest of the time, I have to fight with the icon for VPC
that resides on the host XP machine's task bar. I have to click on it and,
if I am really lucky, that makes the guest machine's task bar cover the
host's so that I truly have VPC in Full Screen mode. Most of the time
though, I have to click repeatedly on the VPC icon in the host's taskbar
making the screen flip back and forth from the host's screen to the guest's
screen and during one of the flips to the guest's screen the guest's taskbar
finally covers the host's taskbar. Usually it takes 5 or more tries to
actually get Full Screen mode for VPC.

This is beginning to be really irritating. I don't have this problem with my
VMWare Workstation machines. Is there some trick that will make the screen
go full screen when I click on Action/full screen? I go back and forth
between virtual machines and the host machine so this problem happens dozens
of times in one day and is quite tiresome.