The current installer locks access to the disk open, so if you did something, like asked it to "format" your disk prior to installing Windows Server 2008, the format process will silently fail, the copy files will silently fail, and you will get a x8007045d during the expanding files portion of the install.
You will need to reboot, choose repair instead of install, click next on the choose OS dialog, then command prompt, and manually format your drive (format c:). You should then be able to continue the installation.
Dan