Yes, this is possible.
First you need to share the PPTP connection. Right-click on the PPTP connection in the Networking pop-up panel and select Properties. Go to the sharing tab and turn on Allow other network users to connect through this computer's Internet connection
Make sure you select the correct Home networking connection for the network adaptor you use to connect to the router.
Secondly you need to route your network traffic from the Linux box through the Windows PC. How you would do that for your version of Linux is beyond the scope of this answer, but the generic command line to do it temporarily would be:
# route delete -net default
# route add -net default gw <ip of the windows machine>
All the traffic destined for the internet from the Linux machine will now pass through the PC and down the VPN.
To undo this repeat as above replacing the IP address of the windows machine with the IP address of the router.
If you don't want all the traffic to go down the VPN but only traffic that is destined for the remote network at the other end of the VPN you can use this instead:
# route add -net <network range of remote>/<netmask for remote> gw <ip of windows pc>
# route add -net 10.1.1.0/24 gw 192.168.1.7
That should then split the traffic from the Linux box - anything destined for (for example) 10.1.1.0 to 10.1.1.255 will be passed on to the PC for forwarding through the VPN. Everything else will continue to go through the router to the internet.