You will need to perform a low level format to remove all data currently contained on your hard drive. You would then want to reinstall your operating system from your recovery partition/hard drive.
Please make sure that you back up any files you need prior to performing any of these steps. If you need additional details on how to format or reinstall your operating system I would recommend contacting either your computer or operating system manufacturer.
If you are only looking to evaluate Illustrator I would encourage you to evaluate the software on a different computer where the trial has not previously been utilized.