I have setup PoPToP on a RH8 box.  All works well from Mac OS X.2 or pre XP computers.  However, the XP computers will connect to VPN, and can communicate to a computer on the LAN providing the amount of traffic being returned is not excessive.  If it is, no data is returned.  I have had a play around with MTU/MRU values to no avail, but reading on the net, it sounds like XP doesn't heed this anymore, or at the least more strict on them. I have confirmed this by setting at up web page with the following:

1)  A web page with the generic Apache web page will not return anything and timesout
2)  A Web page with "Hello There" works fine.

The XP Clients are XP Professional.  I have tried with both SP1 and non SP computers and still no joy.  Is their anyone out there that has this working.  If so, can you please post me your configs on the Linux box that pertain to PoPTop.  I have found a german PDF around setting up on XP Clients, but that doesn't seem to work for me.

Kelvin Smith