Can't allocate available memory to Windows Virtual PC VM on Window

I installed Windows Virtual PC on Windows 7 Home Premium. I have 6GB of
system memory and in task manager is shows 4.3GB available. If I try to
allocate more than about 1GB to a virtual machine (Windows 7 guest) the
virtual machine won't start and I get an error that there are not enough
system resources. If I only allocate about 1GB the VM will start up but I
need to allocate more to it, probably 3GB or so.

I came across a hotfix for a similar sounding problem in Vista
(http://support.microsoft.com/default.aspx/kb/953585) but I'm not sure they
are related and it doesn't help in Windows 7 anyway.

Anyone have any ideas why this is happening? It's a new HP Envy 15 laptop
and it doesn't really have much running on it. I haven't uninstalled the
Norton anti-virus that it came with yet but I shut it down and I get the same
issues.