I'm having this exact problem; it's sending me round the bend!
I've had a really good look around on this site and on Google.
The most annoying thing is I've used the VPN before; from this same network and laptop (when it was running Ubuntu).
- I've turned off IPv6.
- My VPN host doesn't use a domain.
- I'm not running any firewall software and the Microsoft built-in Security Centre is off.
- I've run Anti-Virus & Anti-Spyware software to make sure everythings clean.
- I've tried putting my laptop in my routers DMZ to rule NAT out of the equation.
- I've gone through every running process and killed anything that it looks like it might use/mess with my networking settings.
I'm running Windows 7 Home Premium 32bit.
I really hope someone out there can help me.