Environment varibles

Hi,

I need to define a set of env vars that point to a particular file tree like
so: APP_VER = C:\APP.1.2.3. I then use that var in other vars like so: PATH
= %APP_VER%\bin;... We changes versions quite often, and all I want to need
to do is change the value of APP_VER and the other vars change accordingly.
(I use the System Properties dialog to set the vars.)

The problem is that after a period of time, %APP_VER% gets replaced with
its value in the other vars, i.e. it gets expanded like so:
PATH=C:\APP.1.2.3\bin;...

How does that happen and what can I do to prevent the expansion?

Thank you,
Jim