The lastest for 3.51 is Service Pack 4. It's available at
ftp.microsoft.com where you should look in
/bussys/winnt/winnt-pub/fixes/usa/nt351/ussp4
and then go platform specific (ie: /i386).

You have your choice of downloading one humongous file
(over 10MB) or going to the next directory level to get the
individual files to build a diskette based upgrade.

We've installed SP4 and haven't noted any problems.