It's been a while since I had to do that. If you already have the OS installed on C and if XP is like 2000 and NT, then keep the ntldr, boot.ini, ntdetect.com, ntoskrnl.exe files on your C drive. You may also have to make changes to the boot.ini file to go change your default to point to the H drive and the folder where XP resides.
I suggest you first make a copy of the boot.ini file before you make changes and also not to delete anything until you are sure that the computer boots straight up to XP. There is also a reference in the registry that points to the last known good boot option.
Again, this is what you do for 2000 and NT. XP should not be much different. I have not used XP yet and if you don't know that much with it proceed with caution.
another way might be to just format the C: drive and install XP on the C: drive and then move whatever you want to keep from the H: drive to the c: drive and then delete all files from the h: drive.
Good luck
