Vista Install hangs at "completing installation"

I'm a teacher and I'm trying to install Vista as a dual boot with XP on the first partition. In a room of 25 computers, 4 went perfectly and all the rest hung at "completing installation." All the units are exactly the same: Dual core, 160GB SATA drive, 4GB RAM, DVD-RW, etc. I examined the BIOS in the units that worked and ones that didn't and they're the same. I also tried to change the BIOS to legacy IDE emulation for the SATA drive, but with the same result. The new partition was freshly created and formatted NTFS. What am I missing?? Any ideas would be appreciated.

This will happen frequently due to errors by the DVD drive itself. During the completion phase, at several points, the DVD will be accessed continuously. This will also happen when checking system performance.

In the early days of both floppy and CD drives, it was not uncommon to experience variations in drive performance that were wide enough such that a drive made on one machine could not be read on another. Unfortunately, this has returned as price and quality of drives has been diminished.