How to remove the plastic part on the side of the window?

There are a lot of factors that may influence my answer. If possible, I'd like to see a picture of your window so that I can get a good idea of what you have. Also make sure to take a picture of where the spring would install on the side of the window.

Ah, I see. It looks like you have a builder's grade windows, which means the sashes are not meant to be removed, even if you pop out those small nylon guides. It looks like you have a 3/8" balance that attaches to the side of the window sash near the bottom. If you raise the sash and look underneath, you can see how the balance attaches. Please send a photo of that and I can try to help you find the best replacement balance. I will also need the length of the balance tube.

You won't be able to remove the sash even if you remove the nylon guides. I would leave the guides alone. From your photo, it looks like you have a sash bracket like our 17-062, which is attached when the window is made. For the balance itself, I would recommend our S400-30 non tilt spiral balance with red tip. Our 75-004 tool will also be helpful during installation.

With someone helping you, you will need to drop the new balance in from the top of the sash, screw it in at the top, then let the spiral drop down. You will then raise the sash and, while someone holds the sash in the open position, spin the spiral up to the bottom of the sash. Finally, with our 75-004 tool, give the balance about three turns to charge it and lock it to the end into the sash clip.

Let me know if you have any more questions.