... however this is a system builders disk used on several systems
That's your issue.
Each Windows 7 install disc can only be used to install Windows 7 on a single computer. If you have multiple computers, then you need to purchase multiple install disc whether they be OEM (System Builder) or Retail.
If Windows 7 is validated on all those other computer using the same install disc, then i don't know how you managed to do that.