There is no trial version of Windows 7. You installed a copy you haven't yet paid for.
If you have any qualifying XP or Vista retail copy on hand (which does not need to be installed) you can use Upgrade version key inserted at Computer>Properties activation link, after doing the quick workaround given in Option 3 of
Clean Install with a Upgrade Windows 7 Version to activate upgrade version on a new or cleaned HD.
If you have no qualifying XP or Vista then you must buy a full version key. If this is too expensive you can install Windows 8 free preview version which performs as well or better than Windows 7 with a different GUI that can be modified to the normal Start Menu. Participating as a preview tester can earn you deeply discounted and free copies of the OS as it did us for Windows 7.