Then I assume that your computer has a build in card reader.
For a strange reason the XP setup gives the low drive letters
to the reader's slots even there is no media present while the
setup.
The best solution is to reinstall XP while the card reader
is not present. You can reach this by deactivating USB in
the BIOS setup. If there is no such option then you have
to open the computer and disconnect it from the mainboard...
But you can try to change the letter of the existing Windows
too. Open the registry editor, go to
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
Here you find items like \DosDevices\C:, \DosDevices\F:.
Just change C to F and F to C. Now get an Registy Search
and Replace tool like this:
http://www.articulate-pr.com/RegSearch/REGSRCH.HTM
The replace in the registry
F:\Windows to C:\Windows
F:\Program to C:\Program
F:\Doc to C:\Doc
(or whatever these items are named in your language)
Replacing all F:\ to C:\ might do to much - never tried it.
Then reboot and hope for the best
Greeting from Germany
Uwe