You need an XP CD and when you start the computer, you have a very short time in a Toshiba to right arrow across to the little graphical pile of CDs so you can boot. Run the installation process and when you get to the Format stage, ignore the warning about the existing system and just blow it away.
Once you're up and running, fix the ethernet driver first and immediately update to Service Pack 3 and about 130 subsequent updates.
I suggest while you still have W7 running,. albeit slowly, you get the details of the hardware or at least the network adapters. From their Properties in Device Manager, click Details and scroll to HardwareIDs. Note the four characters after VEN for Vendor and the four after DEV for Device and track down the drivers using http://www.pcidatabase.com to have them ready on a memory stick when the new installation is finished.