VPDN from Win XP-SP2 ws

[erpapi]

We have configured a VPDN in two routers CIsco 1760. Here you have the VPDN configuration we have used on them:

.
.
vpdn enable
!
vpdn-group Group_VPDN
accept-dialin
protocol pptp
virtual-template 1
!
interface Virtual-Template 1
ip unnumbered ser0/0.100
peer default ip address pool VPDN-pool
ppp encrypt mppe auto
ppp authentication chap ms-chap
!
.
ip local pool VPDN-pool 192.168.30.1 192.168.30.14
.
.
ip route 0.0.0.0 0.0.0.0 Serial1/0.100
.
end

These configuration worked very well for many weeks, until we upgraded some PCs to WinXP Service-Pack 2. We have demostraded we can establish VPDN connections to these routers from all ws (including WIN XP-SP1 ws) BUT Win XP-SP2 ones.
We have opened UDP 62515, TCP 10000 and TCP 4500 ports at our Win XP-SP2 ws, but we have never established a VPDN connection from them.
Could any of you give some ideas about this problem ???
Thank You

Enrique