Does it start from stellaris.exe directly? If not, you almost certainly do not have a launcher problem, but a game/Windows issue.
In the game launcher please set Display mode to Borderless.
IF that doesn't help, Please do a full **clean** re-install:
- move any valued save games elsewhere, and user_empire_designs.txt from /Documents/Paradox Interactive/Stellaris/ if you have made any custom empires or races.
- "uninstall" in Steam-Stellaris
- manually delete both the Steam/SteamApps/common/Stellaris AND Documents/Paradox/Stellaris folders
- re-install game in Steam, run a Steam Verify when done.
- add stellaris.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test
In particular you need to make sure there are no files or folders left under either the Steam Stellaris or Documents Stellaris locations, before installing again.
Also if you have FRAPS or the MSI Gaming App or Rivatuner installed, disable it as it clashes with this game.
If that doesn't help:
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach your Documents/Paradox/Stellaris/settings.txt and pdx_settings.txt .
From your Documents/Paradox/Stellaris/logs/ folder, attach system.log .
