One problem might be to get hand on the old installation files for Prime 4. Main reason that versions below 7 are not offered for download anymore is that older versions include the third party muPad (from Mathworks) to do the symbolic calculations. So these versions can't be distributed by PTC anymore because of legal issues.
Another problem might be to get PTC to send you a new license file - unless of course if you have a backup of it stored in a safe place and not only the copy on your broken HD. The license usually bound to the MAC of your NIC, so as long as you are using the same network interface card, your old license file should work OK.
In any case you should try to contact the official PTC support for help.
But it may be that PTC interprets the "permanent" license in a way you won't like ...
The document Luc is referring to clearly states: "PTC will not be able to issue new licenses or provide access to software downloads for end-of-sale versions." Probably not what you expected when you bought a "permanent" license back in the day...
