Vista - Slow browsing inside a VM

In a small training room we have 12 PC's running Virtual PC 2007. Whe have an
XP VM on each of these machines.

When browsing the internet, IE is either dog slow or locks up. Sometimes it
will work well for a while then grinds down. If I close the VM down and just
use the host, IE browses fine. I have checked everything I can but don't
understand what may be happening. Is there a known problem or can you point
to some references that may assist me.

Thanks