"Something on your computer is blocking the VAC System"

[Oregun]

did you try to do the knowledgebase suggestion and repair steam? all i had to do was reboot and my issue was fixed. i had the same message you received. also maybe make sure your AV and firewalls are turned off or make sure steam is on the excluded list for blocking?

 

[v.Kamikaze]

solution: stop wit ur hax! j/k

 

[kaM]

Ok so I found a few things

Uninstall your AV - A disabled AV can still do harm, and honestly AV is useless waste of resources.
Clean your system up (Malwarebytes, Hitman Pro, Combofix)
Start a new account, run TF2, log over to new account (has worked for a lot of people)

Enable DEP - [FIX] An issue with your computer is blocking the VAC system - Steam Users' Forums
This one seems to help most

List of programs that can interfere with steam games - Programs Which May Interfere with Steam - General Troubleshooting - Knowledge Base - Steam Support

Post from someone who fixed it
"Repaired steam service
Enabled kernel integrity
Turned off kernel debugging
Enabled DEP
And also typed "bcdedit.exe /set {current} nx AlwaysOn" in Cmd

THEN I restarted my PC, and ran Steam in admin."

 

[el jorge loco]

I don't know about disabling your AV, though I will agree that some AVs eat up more resources than they should.

 

[IcEWoLF]

Steam has been having a lot of issues as of late.

 

[v.Kamikaze]

i has no AV, lol

 

[Ghonzo]

i has no AV, lol

We got a baller here

I dont know why uninstalling AV will help? I mean Valve/Steam should of known people are going to be using AV on their system...

 

[A-Rich]

Anti-Valve

 

[Spicy]

I've never had issues with steam using AV. It could be something else.