Welcome to the Forum!
Because You are Dual-Booting:
- Are Secure Boot and Fast Boot in BIOS disabled?
- Is Fast Start-Up in Windows disabled?
- Is Your BIOS in UEFI or Legacy Mode?
- What Tool did You use to create the bootable USB Stick?
- What Installation Option did You choose (Install Zorin alongside Windows, Something else)?
Because of the Codecs:
- Did You installed the Codecs of the Gnome Software Center (on the Startpage scroll all the Way down to the End and click on the ''Codecs'' Button)
- Did You installed
ubuntu-restricted-extras? - Did You installed VLC?
- Did You installed ffmpeg?
Because You are using a Nvidia Card:
- Is Your System run in X11/Xorg or Wayland? You can check that in Settings>About. When it is in Wayland, switch to Xorg. To do that, go to the Login Screen (not the Lock Screen). Simply reboot for that. On the Login screen, click on Your Profile so that the Password Field appears. It has to be appeared. when it is appeared, you should see a Gear Icon in the bottom right Corner. Click on it and choose the Option ''Zorin Desktop on Xorg'' and then log in and test it.
- You wrote that You use the 570 Driver. Did You tried it with a Reinstall or the Installation of a different Version? To do that, first type
sudo apt purge nvidia*to delete the current Nvidia Driver. Then typesudo apt install nvidia-driver-[Version-Number] nvidia-dkms-[Version-Number]to install the Driver again or install a different Version (like 550 or 535 for Example). Repeat this to test different Driver Versions.
Then You could try it with a newer Kernel. Instructions and Guides for this, You can find there:
Another Thing: Is in Your Display Settings Fractional Scaling active? If yes, try it with turning it off.
You wrote it is while watching Video's. Is it in a Browser or in a Program? Please name it. And is this the Flatpak Version or the APT Version of the Program. If it is the Flatpak Version, You could try it with the APT Version instead if available.