virtualbox.org

Guest OS: Ubuntu
Host OS: Windows Vista

I have the latest versions of all software including VirtualBox.

I am a web developer and I would like to know if and how to access the localhost of my guest machine from my host's browser. Thank you.

Check the manual. Using Port Forwarding with NAT, or use Host-Only, you can access the machine using either localhost:<port your forwarded>, or the IP address of the VM itself.