It is probably not the motor. I had nearly this EXACT problem in my X5. This is what happened in my X5: my front left window was going up very very very slowly but would come down effortlessly. I thought that it was the regulator. When I pulled the panel off, I saw that the regulator and cables were all fine. So then I thought it was the motor. At that point I figured I'd just swap the entire assembly (regulator and motor) for a new one as I was "already in there." So, I picked one up from a parts recycler. When the part came in, I realized that the recycler sent me the driver's side regulator and motor - after I had already removed the OEM regulator. Needless to say I was upset. To make it worse, because I was going on a 4,000 mile road trip, I didn't have time to send the regulator back and swap it for a new one. Then, in a moment of sheer luck, I caught a glimpse of something I have never seen before...

When you look at the screws you are referring to, you will see that there is an aluminum piece that sandwiches the glass onto the regulator itself (the screw is what pulls everything together). The regulator cables actually connect to this aluminum piece and pull it (and the window) up and down. I noticed that this aluminum piece had completely disintegrated. It is some really cheap pop aluminum and it crumbles apart. So, I thought, "maybe that aluminum piece is the culprit - if it is identical to the one on the driver's regulator and I can swap them over it might fix the problem" - BINGO. I swapped the aluminum piece off my recycled regulator over to my OEM one, put it all back together, and tested it. IT WORKED. The window rolled up with normal speed and force.

In short, what happens is that when this aluminum piece gives way, it causes the window to bind within the regulator and against the window frame/weather stripping. This makes it incredibly difficult for the motor and regulator to move the window up and down.

All that said, I would check to make sure whether that aluminum piece has crumbled - based on your symptoms, and assuming your cables and the plastic reel that hold them are not snapped, it's likely that this same aluminum piece has failed in your car.

*Edit*

I wrote all this in hopes of providing you a part number for that little aluminum piece but, as it turns out, that little aluminum piece is not available by itself - you need a whole new regulator. That said, you could get away with just the regulator and just swap over your OEM motor because your motor is still probably good. If you decide to get a recycled part, just swap in a new regulator and motor for good measure because they will be sold together as a set. (You can pick a regulator and motor up from LKQ for about $75).

Good luck.