Apparently the 480 partition is the first on the drive itself with the 120gb primary as the second. That saw the XP installer place the boot files on the storage partition being first since the installer always looks for the first to begin with while XP was installed on the second. That simply explains why Windows is seeing itself as E not C.
A friend went with the exact model board for a 2006 build while I've been Asus models seeing nForce not VIA or SIS chipsets for several years now. You have the AMI bios there while the NVidia nForce models by Asus see the Award bios.
The basics are still see there for setting the hard drive as the first item in the boot order by highlighting each and pressing the enter key in order to select hard drive or disabled rather then a floppy or optical drive installed. Also make sure that the Plug'n'Play option is set for yes, the LBA mode is set to auto, and with the full screen logo for Asus disabled you can watch how things are progressing during the post tests like what drives are listed, how much memory is installed as that is counted up, and the options for entering the bios setip.
If available the optional boot device menu key shown next to the press DEL now to enter bios setup will also see the press F8 or another assigned F key designated for that there on the first post screen. For Asus boards it's typical to see the F8 key for that while Gigabyte, MSI, and other makes will use the F4, F12, or another key for their optional menu.