Quite some time ago, a customer had forgotten that they were using an evaluation edition of Windows, and they were awakened one morning with the following error message:

The evaluation period for this installation of Windows has expired. This system will shut down in 1 hour. To restore access to this installation of Windows, please upgrade this installation using a licensed distribution of this product.

The customer submitted an urgent request for assistance. "Please advise how we can get this machine working again. We need it to run a demo for a major client later this week."

In the customer's panic, they forgot to read the actual error message, which tells them what they need to do:

To restore access to this installation of Windows, please upgrade this installation using a licensed distribution of this product.

In other words, get a non-evaluation edition of Windows and perform an upgrade install. (Kick it off as soon as you finish booting up so you don't get the rug yanked out from under you if you dawdle and take more than an hour to get through setup.) This will preserve all your existing data while upgrading it from an evaluation edition to the real thing.

Pre-emptive snarky comment: "The existence of evaluation editions with expiration dates proves that Microsoft is evil."