I recently downloaded windows server 2008 r2 trial. It has expired, asks to be activated, but runs fine for me. Is there anything saying I have to activate it, or can I continue to use it unactivated? I know unactivated lacks features, but it has little effect on me. Do I have to activate? The OS is for personal use only. By activate, I mean purchase a Product Key.

Yes you have to activate it

A trial version is just that. Its allowed to let you try it for a time before purchasing. To continue to use it after the trial expires is stealing.If you don't activate it within a time, it will become disabled.