hmm - something like this?
set host=%COMPUTERNAME%
echo %host%
EDIT: expanding on jitter's answer and using a technique in an answer to this question to set an environment variable with the result of running a command line app:
@echo off
hostname.exe > __t.tmp
set /p host=<__t.tmp
del __t.tmp
echo %host%
In either case, 'host' is created as an environment variable.