Did the machine come with a drivers CD? Did the machine come with XP installed? If yest to both, it's probably on the driver disk. Have you installed a new OS on the machine or restored the factory one recently?
If no to cd, borrow a machine, go to Lenovo's site, and download the driver. http://support.lenovo.com/en_US/downloads/detail.page?DocID=DS000699
- Ethernet driver (BroadCom) for Windows 2000, XP - ThinkPad G41
Could the problem possibly be something other than the driver?