Did you run Link Removed before starting upgrade ?
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I'm not sure of what's causing the problem here, but :
1. Your upgrade can stop due to some hardware/driver incompatibility issues.
2. The upgrade media must be appropriate for your vista edition,
vista home -> 7 home
vista business -> 7 professional
vista ultimate -> 7 ultimate
3. Of course both the upgrade media and the dvd drive must not be corrupted/broken
4. For best results, it is worth to reformat the system partition > clean install vista > activate it > do not install any software or drivers before doing the upgrade > upgrade. The idea is to have vista "healthy" before the upgrade, free of any incompatibilities like software, drivers, and modifications.