How to change the default drive letter for my hard drive

Hi all,
I am having a problem with my Windows XP. There are some errors that keeps on popping and my computer seems to run slow. I thought this is caused by some virus, but my anti-virus cannot detect anything. So I decided to format my computer.
But after installing the windows I notice that the default drive is not C: I don’t want to change the drive manually in computer management because it is not safe. How can I change the drive letter back to C? Do I need format it again? And is there any problem if my windows run from a different?
Thanks.
