After I Press Ctrl Key, I can no longer mouse highlight

I had exactly the same problem and eventually found the cause.

In my case, after pressing the Ctrl key, mouse click-and-drag text selection stopped working. The problem occurred not only in Microsoft Word, but also in WordPad and Outlook. Using Ctrl on the Windows on-screen keyboard caused exactly the same problem, so it was not a physical keyboard issue.

After extensive troubleshooting, I discovered several pythonw.exe processes running in the background. When I ended those processes in Task Manager, mouse text selection immediately started working again — without restarting Word.

After restarting Windows, the pythonw.exe processes returned and the problem returned as soon as Ctrl was pressed.

I then checked Windows Task Scheduler and found several scheduled tasks launching Python scripts from unusual folders under C:\ProgramData. After disabling all of those scheduled tasks and restarting Windows, the pythonw.exe processes no longer appeared and the Ctrl/text-selection problem disappeared.

Important: in my case this turned out to be a security issue, not a Microsoft Word problem.

Microsoft Defender later detected Trojan:Win32/ShellcodeRunner!MTB on the computer. The detected threat was quarantined, old infected backup copies were removed, and potentially unwanted applications found during subsequent scans were blocked.

After cleanup and reboot:

• pythonw.exe no longer starts automatically;
• pressing Ctrl no longer breaks mouse text selection;
• Word, WordPad and Outlook work normally;
• Microsoft Defender is able to complete its scans normally.

So if you have this very specific symptom — mouse text selection works until Ctrl is pressed, and Ctrl on the on-screen keyboard causes the same behaviour — I strongly recommend checking Task Manager for unknown background processes and running a full Microsoft Defender scan.

Do not assume that reinstalling Word or changing mouse/keyboard settings will necessarily fix it. In my case Word was only where the symptom was first noticed.

I would also recommend checking Task Scheduler if suspicious processes disappear after being terminated but return after restarting Windows.

Do not simply delete unknown files or scheduled tasks without first identifying them and running a security scan.

I hope this helps someone experiencing the same issue.
:)