Do you mean change it to the relative path or the absolute server path( eg: c:\blah\blah\site )?
I also thought about what Gary and AnonJnr said about the domains/application space. I'm gonna try pointing the domains to an intro page then link to the forum from there. I'll let you know how it goes.
Set your cookie path to domain in the admin options and it won't be a problem. You might also run setup.asp after the change or just reset IIS to clear any settings stored in the application object.
OK. It's fixed! Well, more of a work around really. I pointed the domain to a folder containing an intro page which then linked to the forum in a subfolder. That's certainly good enough for me.
Thankyou very much to everyone who helped with this. It would have taken me far longer without your help. Muchos kudos!
I just noticed, although the sites are working as they should, Site1 has all of Site2's info in "Forum Variables". I guess this is to do with the "application space"??