vbulletin error on windows

Heya

im trying to install vbulletin on a windows system with php enabled. It appears that it has a path problem when trying to install on the root of the domain, it works fine if i use a sub directory. But i want it on the root :-(

i even asked them before purchasing if it works fine on windows and was told a simple yes. ok it does work but like i said, not on the root.

can anyone help? thanks