Semco windows won't stay up

It sounds like you need a new channel balance, though I'm not sure exactly what type you have from this photo. I can tell you that it's a ribbed channel for a tilt window, but to narrow it down further I'll need you to remove the sash from the frame and take a clearer picture of the rest of the balance. I need to see the shoe, specifically.

In the meantime, you can see our selection of ribbed channel balances here. Note that these aren't the only options, just what come standard on some windows. There are many, many custom configurations out there. That's why it's important that we see the whole thing.

Yes, you would need to replace the entire balance. We should be able to supply the part you need but as Tom mentioned, we'll need to see an undamaged balance fully removed to identify the shoe attached to the bottom of the metal channel. That should tell us what replacement balance would be right for your window. 

These photos don't tell me anything new, unfortunately. There is one key detail that's missing, which is the shoe at the bottom end. There are many, many different varieties out there and I don't know what your balance has. Without seeing that shoe, we cannot help.

The video below may be of some assistance, in terms of removing and replacing them. I don't know if the balance in this video is a match for what you have now, but the principle of removal should be the same.