This is utterly simple and mostly you did it with the mouse.
To do it with command line and powershell, proceed this way.
Open a powershell window
or if you have no right to edit the machine-wide environment, runas admin
runas /user:yourdomain\youruserplusadmin powershell
then access the environment assembly
[environment]::setEnvironmentVariable("PATH","c:\oracle\product\11.2.0\client_1\bin;C:\WINDOWS\;C:\WINDOWS\system32;C:\Program Files\Putty;C:\Program Files\Perforce;C:\Program Files\TortoiseSVN\bin;C:\WINDOWS\system32\WindowsPowerShell\v1.0","MACHINE")
MACHINE is a for a Machine-wide setting and USER is for a USER-wide setting.
I hope it will be of some use
Completly unrelated, Enterprise Manager 12c for AIX is out. Do not wait it for HPUX, it is not planned. For Windows 64bit, it will come “between March and May 2012” according to note 793512.1 on metalink