There is no concept of split tunnelling in PPTP. PPTP is a Point To Point Protocol (hence the name), meaning all traffic goes from one point to another.

There is a way around it but it's pretty ugly. In the Dial-Up Networking connection entry on the PC uncheck the "Use default gateway on remote network" option. Then add a route to your PC for the network behind the PIX, the next hop for this route is the IP address that the PIX assigned to the PC when the tunnel came up. Since you can't assign a specific IP address to a specific client, you have to manually add this route in each time the tunnel is brought up (as I said, ugly).