I currently run windows XP on a Dimension 9200. When I run the Windows 8 upgrade assistant it indicates that:
Secure Boot isn't compatible with your PC- Your PC's firmware doesn't support Secure Boot so you won't be able to use it in Windows 8.
What does this mean and what are the implications
It's not that big of a deal. Secure Boot simply protects your PC from certain types of malware. Only newer Windows 8-certified hardware will be equipped with it.