There have been many different suggested solutions in the Steam forums for this exact issue, and some of the solutions seem to have worked for a minor subset of the users facing this problem. I'm sure you've been through those forums and tried every sensible suggestion. Just like myself.
In my case, however, none of the solutions did work. The problem I was having was that I was using the same computer for both playing CS:GO, and my software development work. I had tons of development tools installed, and some of the stuff I had installed was somehow hooking into the game process, or otherwise running in the background and being picked up by the useless and incompetent tool we call VAC.
This particular message is displayed when VAC cannot verify your game installation (the files, or the running process) for whatever reason, or when it detects a suspicious process in the background from its own weird perspective. It doesn't detect a cheat per se, but it just detects that there's some kind of modification going on, or a suspected modification that it deems as possibly going on.
At first, I used to format the PC and install everything from scratch, and it used to work for a while. But eventually I would start getting the same issue.
Basically in order to resolve it for good, I've started using a separate PC for gaming, and never had the same problem again. (It's been years.)
I realize this isn't the ideal solution you're looking for, but your case might be similar... Some software development tools hooking into other processes, some badly designed driver the game is accessing, the antivirus software being way too eager than it should, a virus or a trojan... I've even met someone who had the very same issue just because he had a PowerShell update installed on the machine.
What you can try before going into extremes is:
- Make sure your antivirus software (I am simply assuming there is one installed) is up to date, and perform a full scan.
- If nothing shows up, pick a few well known alternative antivirus suits, and install them with their trial licenses one by one, performing a full system scan with each one of them. These first 2 steps are just to insure there's no virus/malware messing with your system.
- Check which applications are running on startup via the settings or the task manager. You can disable anything that you don't actually need.
- If that doesn't work, try disabling everything, and give it a try. If you're good, then try enabling them one by one in order to find out which particular application is causing the issue.
- If that doesn't help, you may wanna try checking out what's starting at boot or login time via CCleaner. And if it shows up any extra applications, you can try the same steps usinc CCleaner as well.
- Still no luck? One last option you might try is uninstalling every single application completely, apart from Steam and drivers. Take a note of the applications you've just uninstalled. If this actually works, you may want to install them in order of actual necessity, one by one, and check if that particular one is the problematic app.
If none of the above helps you, I'm afraid you're gonna need a clean installation of windows.
Of course, having a PC just for gaming isn't something everyone would be willing to or able to fork out some extra cash for. But you might want to try having 2 separate Windows installations on the same machine, one for just gaming, one for everything else. That would greatly reduce the risk of experiencing the same issue in the future.