First, you should set up a proper dual boot. Boot into Win 7 and use bcdedit to add the XP home as a boot choice and save. You may have to first assign a drive letter to your XP partition from within 7 if you haven't done so already.
Now when you boot up, you will be presented a choice for which OS you want, provided you added the XP correctly. This is if you leave your bios boot to the 7 drive.
If after doing this, you boot to the XP drive, it will only load XP with no choices.
Now as far as your connection is concerned in Win 7. Disable IPv6 if you do not plan on using the Homegroup feature and do plan on sharing with the classic Windows' method. If you do disable IPv6, also disable the IP Helper service.
Set a manual ip for the nic.
IP = same as router config page, only changing the last part to any number between 1-255 that isn't being used by any devices.
Subnet mask = 255.255.255.0
Default gateway = router's config page ip address. Usually 192.168.0.1 or 192.168.1.1.
DNS Server = same as gateway
Scan your system with Malwarebytes to ensure it is clear of any funk. Also make sure the mdnsresponder.exe is not in task manager and the Bonjour service is not installed on your machine. You do not want Bonjour lol. Check in services and in program files.
You could either temporarily or permanently disable the DNS Client service.
Open you hosts file and make sure nothing weird is going on there.
\etc\hosts, <That's for x86. Not sure about x64.