Vista - ICS problems

Hello,

I've been having some problems with Internet Connection Sharing ever since I installed this new Windows Vista Business SP1 (x64) system as a replacement for a Windows XP Professional system. This new one is connected peer-to-peer to another Windows XP Professional computer in anothe room. Although I've managed to get the whole IP assigning thing sorted out, and the Windows XP computer seems to see that there's an internet gateway, it can't even ping the Vista computer, and there's certainly no internet on the XP computer.

What could the problem be? Thanks in advance for any help.

FYI, here's the ipconfig info from both the computers. The XP config is in red and the Vista is in blue. (The reason the XP config looks a bit weird is because I wrote the settings down rather than copying and pasting them.)

Windows IP Configuration
Host Name: office
Primary DNS suffix:
Node Type: unknown
IP routing enabled: No
WINS proxy enabled: No

Ethernet adapter Local Area Connection 3:
Connection specific DNS suffix:
Description: Marvell Yukon 88E8053 PCI-E Gigabit Ethernet Controller
Physical Address: 00-13-D3-E7-48-B4
Dhcp Enabled: Yes
Autoconfiguration Enabled: Yes
Autoconfiguration IP address: 192.168.1.56
Subnet Mask: 255.255.255.0
Default Gateway: 192.168.1.5

* * *
Microsoft Windows [Version 6.0.6001]
Copyright (c) 2006 Microsoft Corporation. All rights reserved.
C:\Users\something>ipconfig /all
Windows IP Configuration
Host Name . . . . . . . . . . . . : QuadCore
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Mixed
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : home
Ethernet adapter Local Area Connection 2:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Realtek RTL8139/810x Family Fast Ethernet
NIC
Physical Address. . . . . . . . . : 00-0E-2E-83-D3-2F
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 192.168.1.5(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1
NetBIOS over Tcpip. . . . . . . . : Enabled
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : home
Description . . . . . . . . . . . : Realtek RTL8168B/8111B Family PCI-E Gigab
it Ethernet NIC (NDIS 6.0)
Physical Address. . . . . . . . . : 00-1F-D0-57-04-EA
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::f02b:2ddf:b78e:d34b%10(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.3(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 28 August 2008 09:27:23
Lease Expires . . . . . . . . . . : 29 August 2008 09:27:53
Default Gateway . . . . . . . . . : 192.168.1.1
DHCP Server . . . . . . . . . . . : 192.168.1.1
DNS Servers . . . . . . . . . . . : 192.168.1.1
NetBIOS over Tcpip. . . . . . . . : Enabled
Tunnel adapter Local Area Connection* 6:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : isatap.{046F1E4C-1644-4107-88DD-A7F9211D3
1A3}
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Tunnel adapter Local Area Connection* 7:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : home
Description . . . . . . . . . . . : isatap.home
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Tunnel adapter Local Area Connection* 11:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
Physical Address. . . . . . . . . : 02-00-54-55-4E-01
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Tunnel adapter Local Area Connection* 12:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : 6TO4 Adapter
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes