Yeah, that should be 255.255.255.0, I'd say that because it's got you on a subnet by yourself that's why you can't access the shares. Either that or the route option you configured isn't working properly.
I don't know enough about pptp to tell you how to change it but you could try changing it manually on the windows machine.
I brought my pc to the place where the PPTP server is located. Inside the local network the VPN works perfectly. So I decided to use my cellphone as a 3G mobilehotspot to connect to the VPN from the internet and... it works! I brought my pc back to my house and connected to the vpn with the same metodology (both through my router and my cellphone 3g) and it doesn't work! How is that possible?
Finally found the solution!
Actually vpn both on server and client side was perfect since the beginning.
For some unexplicable reason I don't know CUPS server was creating some net issue. I only needed to disable it to make the vpn work flawlessly!
Thank you for the support though, I learned something.
S