Changing Drive Letter from I: to C:

Try creating a new share of \\computer\c$ and assigning it a drive letter of c:.

Sorry, that' a new share of \\computer\I$.  You would need to reinstall windows otherwise because all the registry keys are mapped to the I drive.

Instead of subst - use junction: http://www.microsoft.com/technet/sysinternals/FileAndDisk/Junction.mspx

Third-party control over junction: http://www.rekenwonder.com/linkmagic.htm

Does the computer boot Windows from a drive called "I"?   If that is the case, there is not much alternative to reinstalling Windows.  There is a Microsoft fix that is supposed to allow you to reassign the boot drive letter:

http://support.microsoft.com/kb/223188

I have never managed to make this proceedure work - whenever I have tried it, I have ended up with a non-booting computer.  There are lots of good reasons for calling your boot drive "C" , and very few good reasons to call it anything else...

If you have a large enough spare drive you could try a drive cloning app. There are some freeware ones out there. I personally use Acronis True Image. Make a backup image then using the restore partition allows you to make the restored partition Active and assign it a drive letter. I have never tried restoring an image of the boot partition to a different drive letter though.
I concur with phototropic that there are few, if any, good reasons for installing Windows to a drive letter other than C though most programs use the environment variable %systemdrive% which will be whever you installed Windows there are always those old or poorly written apps that hard code C:

here a page on how to do it :
http://www.petri.co.il/change_system_drive_letter_in_windows_xp.htm

This is not a recommended procedure - as tcicatelli suggests - reinstall windows from scratch - otherwise you may hit bigger issues down the line.

nobus,

Have you successfully applied this proceedure?  I have tried it twice, and both times, as I said above, I got a non-booting pc.  Other techs I have spoken to have reported the same thing...

fcruz5,

You should be aware that the Microsoft Kb article referenced above is not tried and tested...

no i was lucky enough not to need it