%ProgramFiles% i think is the variable you're looking for.
you could check what variables there are by going to a command prompt, and typing SET
it lists all variables active on the system. Everything on the left is the variable name so just put % around it and your good.
and you may want to use /Y as there should be a winamp.ini already there.
and you may also want to copy it to the windows directory as there is a copy of one there too.