ok bootable disk = the disk with windows on it, or a hard drive with windows installed. the bios has a set order of places to check for that, and you can set the order in the bios.
ex. 1. hard disk
2. cd-rom
3. floppy
this means it will check for windows on your hard disks if it is not there then it will continue to check for a window cd to install to your hard drive, and then your floppy to get an old windows version or something like that.
A: means its looking for a floppy, just check floppy off as last and cd as first, with the hard disk in the middle. thats what i do unless you already have windows on there and u want startup to be like 5 seconds or so faster, then check the hard disk first.