|There is generally not problem using an old hard drive in a newer computer as a second hard drive. Normally you just plug it in, turn the system on, save any files that are on the old hard drive that you may want onto the main drive, then format the hard drive to remove EVERYTHING on it. When the format is complete, you can transfer files over from the primary drive to this secondary hard drive to free up some space on the primary drive. You will NOT be able to move programs unless you uninstall them, and then reinstall them (select Custom install and select the second drive to install on).|
I said generally not a problem, but you will need to see what type of hard drive each computer has as far as the interface (SATA or PATA(IDE)), and if they are different, if the newer computer supports the other type. As an example, if both are SATA then it is easy as in you just plug it in. If the newer computer is SATA and the older has an IDE interface, you will need to see if the new motherboard also has the IDE port as well. If it does, it is possible, if it does not, then you could not without purchasing a PCI interface card for it. In that case, being that a new SATA drive would not be that much more than the PCI card would be, I would just recommend a new drive instead.
You have to be a little bit crazy to keep you from going insane.