I have used a three key approach to securing an application. There are two keys that are provided in the application; the third is determined from the client PC (the serial number of the harddrive or CPU). The first two keys are an internal and external key maintained in a table that is included in the application (not distributed separately but compiled into the .app/.exe file).

When the program is started for the first time, the program randomly selects a set of keys from the table. The external key is combined with the PC serial number and then encrypted and stored into the registry. To register the program, the user provides the external key (now read from the registry) displayed on a registration dialog and then enters the internal key provided by me. Now the three keys are stored (internal, external and serial number) into the registry by encryption (you could also store the current date). Each time the program is started, these are checked for matching against the internal table and actual PC hardware; if any is not matched, then the installation registration is invalid and the keys are erased from the registry with the process starting over for a new registration.

This prevents a user from simply copying the registry between PCs (the serial numbers will not match). Also, you can embed the date of registration and then calculate a expiration date.