How can I utilize this portal for my HTML Pages

[Pc]

I want to be able to do: http://mysite/?page=help.htm
or:
http://mysite/?page=info.htm
I want to keep the links and logo the portal includes, only put the html file in the middle. I know this is possible with ?page=2, etc. but I want it to do like ?page=info.htm and it will include info.htm.

[odius]

must be an easy 'if url "this" then' for that

[Imajica]

I wound up using an iframe for that

unless someone has found a better method