If you want to share the same wordpress installation for multiple sites/blogs, I’d suggest looking at WordPress Networks. And if you need different domain names (and not just subpath or subdomain) you could go with the plugin called WordPress MU Domain Mapping.
If you have lower level access to your server (and not just a shared hosting) then you could probably create a link to the wordpress installation from all your sites root, but that would just open up for so many problems.