Windows Vista already contains driver(s) for RealTek Ethernet. You can However install/update custom too. If your ethernet is
RTL8139(A/B/C/D/8130)/810X series click
HERE to get the latest version of driver.
Visit *www.realtek.com.tw/downloads/downloads.aspx for any further information.
P.S. Since Windows Vista haven't been launched officialy hence there's no official Vista supported driver(s) avalible. But It's come to my notice that many of XP's driver(s) work same in Vista too so download that latest driver and install it. It should work properly.
Cheers