I noticed a few other people were bitten by the 1400 byte MTU in XP 
with PPTP and m0n0 so here's the official Windows KB article on how to 
change the MTU on XP

http://support.microsoft.com/?kbid=826159

This will only change it for VPN connections on XP which is what you 
want rather than changing for a particular device like some articles on 
the web will have you do.