You will not have access to critical security patches or the latest feature enhancements provided in official service packs.
If you need to use PTC Creo 8, here are the official ways to obtain and license the software:
Searching for a "license key" or "full version" of PTC Creo 8.0.0.0 for "offline" use typically points toward unofficial or cracked software. Using such versions carries significant risks and lacks the benefits of a legitimate license. Legitimate Access to PTC Creo 8