Ah! I had this same problem for the last two days.... just fixed it. \m/
First of all use static IPs only in both connections,
1. Open your LAN TCP/IP settings properties
2. Go to WINS tab
3. Choose Enable NETBIOS option. This shud help you make static IP association possible in LAN when a dial-up is present.
Sorry for typing this info in a hurry.. will repost if not clear
Cheers..