On Toshiba's support site in Asia:
http://pc.toshiba-asia.com/sg/support/drivers
Fill in Products: Notebook Computer, Series: Satellite, Models: M50, OD: WinXP, then Ethernet LAN. I hope you can get them; I'm having a problem with that site at the moment.
Was there a driver CD that came with your PC? And are you _certain_ that it is a driver issue? If it worked before and stopped working for no apparent reason, it may not be a driver. If you re-installed the OS, then it does need a driver before it will work.
See this thread for a good post on the order of installing drivers: http://forums.toshiba.com/t5/Drivers-and-Utilities/Drivers-for-M50-WinXP/td-p/128839