If one laptop works but no other one, it could be the router limitation. These search result may help,

VPN Issues
     Some routers may take just one VPN connection The connection to <connection_name> made by user <user_name> using device <com_port> was disconnected. ...
     www.chicagotech.net/vpn.htm  

Laptop can't VPN while desktop can
     Some routers may take just one VPN connection ... Cause: Some router takes only one connection. VPN connection appears with a red X ... ...
     www.chicagotech.net/VPN/vpnrouter.htm  

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 
 Hi,

 I have two almost identical laptops running XP that connect to their
 owners (different) networks using VPN and PPTP. We recently replaced
 out trusty netgear wireless router, once it started going doolally.

 The replacement was a belkin  F5D8630 (pre-n, adsl modem, wireless).
 However, both laptops in question are running 802.11g crads (not the
 proprietry belkin pre-n cards)

 All of a sudden, with the new router, one of the PC's can't maintain a
 reliably VPN connection (for more than 30  seconds). However, the other
 is still absolutely fine. The wireless connection itself seems fine
 through our router, indeed it is a noticeably stronger signal with
 greater range than its predecessor, so it doesn't seem to be the
 wireless connection itsef that is the problem.

 This initially made me suspect the problem lay with the host networks,
 and the timing was coincidence. However, if we use a neighbours
 wireless network both PC's can connect reliably to their respective
 VPN's.

 Belkin suggested port forwarding, but that is no good becuase firstly
 we have lots of PC's needing a VPN, and secondly with DHCP and changing
 numbers of laptops port forwarding would never work.

 Can someone explain to me what may be going on now that wasn't
 previously, and still isn't with our neighbours router? I thought it
 might be that there was no PPTP passthrough, but in that case surely no
 laptops should work.

 Any ideas? I'm getting shouted at a lot till I can find a solution.
 Even if that solution is to change the router, it would be nice to know
 what was wrong in the first place...

 Cheers,

 Ben

Hi Ben

Not sure if you have fixed this or its no longer relevant but I had similar
issue. Turns out the microsoft outlook connection management was set to drop
dial up connections after checking for new mail, needless to say Outlook
spotted that there was a VPN connection which it proceeded to close without
any warning...

Might be something similar on the specific PC.

If you think this is worth following up please post a reply and I shall
write up the use I made of the Microsoft PPTP logging function and Ethereal
(network protocol analyser software).

Cheers
Steve