This is an odd one. One of our laptops is unable to connect to any PPTP
vpns, it bombs out with a 721 error.. It can still connect to ipsec
vpns. It cannot connect to pptp vpns over any of the net connections
available to it (broadband/3G/wireless/GPRS/modem)

Turning off the XP firewall makes no difference.

The pptp used to work on this laptop, but stopped working some point in
the last few weeks. I thought it might be the result of a windows
update, but other laptops that have had the same updates can connect.

I'd be grateful for any hints or tips as to where to go from here.

here's the output from the pptp client... nothing unusual.

Initializing WinSock...
Obtaining host information...
Successfully resolved server's host information

======================================
Enter data to send to server (between 1 and 255 chrs.), then hit enter:
-->test

Successfully connected to server using TCP port 1723 (PPTP)
Sending data to server

Waiting for a reply to the data which was just sent...
Received a reply.  Reply contains the following text:
--->

=================================
Connectivity test to TCP Port 1723 was successful!!!
Closing down socket...
=================================

Creating a socket to test GRE protocol traffic...

Total GRE packets sent = 1
Total GRE packets sent = 2
Total GRE packets sent = 3
Total GRE packets sent = 4
Total GRE packets sent = 5

=====================================
Check server to see if the GRE packets were received successfully
=====================================

Closing down socket

Goodbye!

Sounds like GRE issue. However, since you have tried many different connections and other laptops work, I would double-check the laptop settings. Do you have other security software like Onecare running on the laptop?

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com
  This is an odd one. One of our laptops is unable to connect to any PPTP
  vpns, it bombs out with a 721 error.. It can still connect to ipsec
  vpns. It cannot connect to pptp vpns over any of the net connections
  available to it (broadband/3G/wireless/GPRS/modem)

  Turning off the XP firewall makes no difference.

  The pptp used to work on this laptop, but stopped working some point in
  the last few weeks. I thought it might be the result of a windows
  update, but other laptops that have had the same updates can connect.

  I'd be grateful for any hints or tips as to where to go from here.

  here's the output from the pptp client... nothing unusual.

  Initializing WinSock...
  Obtaining host information...
  Successfully resolved server's host information

  ======================================
  Enter data to send to server (between 1 and 255 chrs.), then hit enter:
  -->test

  Successfully connected to server using TCP port 1723 (PPTP)
  Sending data to server

  Waiting for a reply to the data which was just sent...
  Received a reply.  Reply contains the following text:
  --->

  =================================
  Connectivity test to TCP Port 1723 was successful!!!
  Closing down socket...
  =================================

  Creating a socket to test GRE protocol traffic...

  Total GRE packets sent = 1
  Total GRE packets sent = 2
  Total GRE packets sent = 3
  Total GRE packets sent = 4
  Total GRE packets sent = 5

  =====================================
  Check server to see if the GRE packets were received successfully
  =====================================

  Closing down socket

  Goodbye!

Limited or no connectivity (tried everything on pchuck's, out of ideas)
Laptop can't join/rejoin workgroup
one way share
Dialup internet access strangely intermittent
A virus has disabled my internet connection
Alright...
Network PC's for Printer Sharing Only
accessing shares
Big problem!
Networking 2 different networks