It sounds like you are attempting to connect an additional system disk (original), in parallel with your current (new), system disk.
If you're lucky, it may be a simple case of doing the following (under XP):
1 ) Right-click 'My Computer'
2 ) Select 'Manage'
3 ) Click on 'Disk Management'
4 ) You should be able to see the original disk in the list (check by size), with or without a drive letter, if not someone else will be along shortly
5 ) Right-click the original disk
6 ) Select 'Change drive letter and paths'
7 ) Click 'Change'
8 ) Change the drive letter to any free letter under 'Assign the following drive letter'
9 ) Okay everything and open 'My Computer' to check the change has taken
Ideally you should then transfer/burn any files that you want to keep and format the drive.
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