Ok, I was not clear :) sorry.
First of all, Don't set ORACLE_HOME variable, ok?
Second: there is a shortcut in Start=>Programs=>Oracle Installation Products=>Home Selector
This little program does what I say, it changes in the PATH the order of appearance of any Oracle Home, so a program like sqlplus will run from C:\ORANT\bin or C:\Oracle\Ora81\bin whichever is first in the Path.