You're welcome and thank you for the medal.
If you have access to the Mathematica 8 program, then this problem could have been solved using the Constrained Optimization function, Minimize:
\[\text{Minimize}[\{2h+w,w h==20000,w>0,h>0\},\{w,h\}] \]\[\{400,\{w\to 200,h\to 100\}\} \] The 400 is the minimum perimeter value.
No knowledge of the Calculus required with this approach.