How to use timer without freezing FORM

Using a timer won't "freeze a form" unless you are doing something silly: either a very, very short Interval or a long running method in the timer Tick handler method.

Timers aren't "magic bullets" - they are events that are fed into your code at a specific interval (or at least, after the interval) so that you code can do something that needs to be done often, like update a time or countdown on the screen, reflect the cursor position on a status line, that kind of thing. And as such, it runs on the main thread, which also processes everything else for the form. If your time plus the other processing takes too long, the Paint events never "reach the front of the queue" so yoru form never updates, and in extreme cases your user clicks are never processed either.

If you want to do a long running task, even if you want it to run at a specific interval you need to move it into a background thread and update the UI only when it has results. As CPallini suggests, a BackgroundWorker is a good choice for this as it includes a mechanism to report progress / results back to the mind thread - which is the only thread that can access UI components at all!