No, the trial is limited to 90 days before you must subscribe.
If you reinstall OneCare on the PC you will get 15 days before you must activate or subscribe.
If you format the hard drive, reinstall Windows and then install OneCare, you can start a new 90 day trial period, but that sure seems like an awful lot of trouble to avoid paying for a subscription.
Why do you need/want to extend your trial period?
-steve