The problem is likely a faulty dimmer switch. Here are step by step instructions for fixing it. (Try it - it's easy and way cheaper than having a mechanic fix it).
The Ford Focus high-beams (won't shut off) problem is very common in 2000-'05 models. So common, in fact, there should have been a recall, but there wasn't. Luckily, it is also VERY easy to fix. This was the first time I ever fixed anything other than a bulb on my car and it took me less than 30 minutes. Here's how:
First, buy the part. The "dimmer switch" is the lever to the left of your steering wheel that controls high/low-beams and the turn signals. You can get it at Pep Boys, Standard Autoparts, or a similar store for about $45.
Now, to install the part: Make sure the car is off as are the lights, radio, etc. First, disconnect the clamp connecting the negative side of your battery to the car. To do this, look under the hood and take the plastic cover off the battery. Use a socket wrench to loosen the nut that is holding the clamp to the negative part of the battery (in my '04 Focus, the negative side is on the right when you are facing the battery from the front side of the car). Once the clamp is loosened, yank the clamp upward until it disconnects (this might take a little elbow grease).
Now, go back to the driver's seat. You are going to take the following steps to remove the switch and replace it with your new switch: First, notice the tabs on the left and right of the steering column. Use a flat-head screwdriver to push them in and release the top cover of the steering column. The cover won't come off the dash, but now you can open it up to see inside the steering column. You will see the switch you are going to replace. Press the tab (on the back end of the switch) to release it. (You will hear a "click" and the switch will adjust upward once you press the tab). NOTE: You may need a screwdriver to help you work in the steering column, unless you have small hands. Now, slide the switch up from where it is held, but be mindful that it is still connected (by wires) to the interior of the column. The next part was the toughest for me, but it really is very simple. You need to remove the grey plug from the switch. It took me some pressing with the screwdriver and really working the angles, but in the end I think it was sheer brute force that got it disconnected. Now the switch is out (and you feel awesome - even with bloody knuckles).
All that is left is to put the new switch in ... and that's REALLY easy. Connect the new switch to the grey plug that you just disconnected from the old switch. Mindfully slide the new switch into the spot where the old switch sat. Push it in so you hear that same "click" when the tab you had pressed to release the old part now secures the new part into place. Once it's in, you are all set. All that is left to do is put the steering column cover back in place and reconnect the battery.
Now, isn't that better than blinding everyone on the other side of the road?
If this helped you, let me know.