WinXP installed on drive F: !!

Hello,
I just installed XP on a ASUS P4P800-E motherboard, and I'm surprised to see the OS is installed on Drive F: (not C .
My configuration :
A: Floppy Drive
B: nothing
C: USB card reader 1 (connected to MB's internal USB port)
D: USB card reader 2 (connected to MB's internal USB port)
E: CD-ROM (ATAPI port)
F: Hard-disk (Serial-ATA)

The 2 USB card readers are (physically) inside the floppy drive, so I connected them to the MB's internal USB ports.

1. Can OS on drive F: be a problem ??
I'm worried that the USB ports are listed before the HDD. So if I add later an USB device, my HDD might become actually drive G:
2. How to force the HDD to C: ??

Last time I build my PC, there was no S-ATA nor USB drives !! I'm lost. Thanks for advice.
Phil