Windows XP VPN client with Firebox X500?

My remote users have always used the default Windows XP VPN client to connect
to the office network via Windows 2003 VPN.

Last week, we installed & configured a Watchguard Firebox X500 w/ MUVPN
(Mobile User VPN). I want to continue to use the default XP VPN and not have
to deploy a third party VPN client to my remote XP users.

I'm able to successfully establish VPN connectivity and remotely administer
the server via Remote Desktop Connection (which is all I really need). Some
of my clients however, are telling me that working via remote destop has
become painfully slow

I'm thinking that once successful connectivity is established, the actual
VPN client used makes little or no difference. Others are suggesting we
reconfigure all mobile users' laptops to use a Watchguard VPN client.

Is anyone here succesfully using the default Windows XP VPN client to access
a network behind a Firebox? ????