You should find an item named hard drive under the onboard devices or integrated peripherals section in the bios. You simply highlight that and press the enter to see another screen come up showing all hard drives and even in some cases card reader will be thrown by the manufacturer.
The #1 drive there besides an ide drive is generally numbered by the port the drive is plugged into. Now with only one hard drive plugged in at the moment you will see that automatcally being the default when first in the boot order as well. If you find the list screen you'll see for yourself that will be the default at the top and only drive shown.
With an infection being known to be present you will want to scan everything on all drives to prevent reinfection in case files backed on a separate were effected. Running the latest AVG 8.0 and even bouncing between that and others will likely nail the bug for you while you wait for help on the log.
Once the backed files are gone through and safe on one drive/partition simply wipe the current XP primary for a clean start since you will be performing a full install anyways for the drive letter correction. You "kill two birds with one stone" is the common expression there.