It depends on the particular cracktivation they have run.
If it is a boot loader, they need to use the uninstaller. Then they can try inserting the new key at Computer>Properties activation link.
If this fails, run a Repair Install
which resets activation.
Finally, it may require wiping the HD for a clean reinstall to remove all remnants of the crack.
Any modding of the BIOS needs to be repaired by flashing the latest update version from maker's Downloads webpage.