Thanks - it kind of put me in the right direction. I didnt realise the re-write module added itself under IIS.
What I was able to do was upload a web.config file for wordpress in the root of my website.
Upload a WebWiz web.config file in the /forum folder on my website
Run IIS manager, browsed to /forum folder, run the URL Rewrite module and disable the wordpress rule only for the /forum folder.
Fingers crossed all seems to be working ok now.
I have no idea how to manually write a web.config file to sort this out mind...
Thanks