It's isn't that simple anymore, that's why the drive letter changing is not applicable for a bootup drive
unless XP was originally installed as C:\. It's quite a mess now, and you're lucky you can still access the drive. I doubt the software on it will still work though, but that depends on how deep the change was.
In either case, have the system backed up after making it slave. Do this at first on a separate drive. You can attempt to first restore the drive letter whist having your HDD as Slave on your sisters computer and going into Start>Run>type:
diskmgmt.msc hit Enter.
Click on the drive and choose "change drive letter and path". Make sure to put it back to what it was before, i.e. "I:\". Then try putting it back in your system and booting from it to see if it works. If not you'll have to attempt the rest of the serious methods outlined below:
If you can, bootup using something like the free bootup XP Disks, then they will allow to get your system started and startup RegEdit or any third party registry editor to revert the changes.
You can also use BartPE or better UBCD4Win to boot from and try this method. Whether they will allow access to the Windows registry is something I doubt though, as it'll be unloaded and there will be many bits and bats unchanged that way.
However, here are the Microsoft support methods, which are much more reliable:-
This is the complete one requiring
many steps:
http://support.microsoft.com/kb/307545
The base MS files here are changed. But it's the best shot at complete recovery.
And this is the short hand to try before the above:
http://support.microsoft.com/kb/304449
See how they go. :smile: