Change partition letter to C - installed as H

When I installed Windows XP Home the drive letter was H (C was being used). I have since renamed all other partitions and now would like to change H to C. When I try to do this I get the message: This partition contains the windows directory. If you change the drive letter, your system may not boot.  Where can I find instructions on how to go about changing drive letter back to C and having my system boot to C. Thank you for any and all suggestions.

Peggie