I've tried to avoid suggesting the latter, because I do NOT want to sound like a 'nix fanboy. That whole "win sux, 'nix rules" thing gets very tired, real fast. But it needs to be said that the unix equivalent of what you're running at the moment is considerably more robust and stable. (Being free is also pretty nice too!)
May I recommend
Ubuntu Linux Server Edition. (L.A.M.P. edition which is
Linux,
Apache,
Mysql and
PHP already installed and ready to go upon installation...)
You could literally be up and running in less than an hour.
Why not
try it in a virtual environment like VirtualBox or VMware first so you don't have to commit the entire system to it in case of drama. I've run vbulletin successfully under this exact configuration for a long time. Not under the same load that this board takes at times - but it still zero issues and very LOW server load.
Just a suggestion... because it's definitely be a step up from the troubles you're experiencing now and will not require any additional financial outlay (licensing etc.)
-snake
but we will definitely be moving to a linux system eventually. Debian is a possibility, as is Fedora (the linux distros I am most familiar with).