Nothing directly in mojoPortal code is trying to write to the windows/temp folder, things like that may be done by the .NET runtime or ASP.NET compiler, but we are just using framework classes to serialize to xml.
You are correct that if you use that config setting it disables the indexing of content into the search index, it disables it for all features not just the forums. I would expect it to happen when you edit any content that is indexed.
Googleing for this error I find this
It seems only your host can solve this problem.
Hope it helps,