the desktop responds slowly when the laptop drive is connected? how did you connect it? USB or internal? is this a SATA or IDE laptop drive?
does this machine have a restore disc to reinstall Vista? Perhaps there was an issue with copying the image. Also, perhaps the drive is acting funny. From your description, I would place that type of behavior into the "failing drive" category. Ensuring that you have the proper "slave" configuration also could be an issue. SATA isn't master/slave. IDE typically won't let you see the drive unless you have the jumpers right. If the jumpers aren't right, then you could see slow performance from a desktop. Also, if it is an IDE laptop drive, desktops aren't setup to run a laptop drive. You may need 2.5" ide USB housing to check the drive.
Also, check the manufacturer on the drive. You can usually get a boot disc to run on the laptop to test the drive's capabilities. Read/Write/surface area is what most commonly is tested. If you can find the tools for it, I would low level the drive and re-run the Vista installation process. Perhaps there is data that isn't being removed properly or the platter is keeping a magnetism that is causing the drive not to copy information. This is an extreme case. Usually the tools will tell you exactly what is wrong with the drive, if anything.