Originally posted by StreamBG:

This fix is tested and works 100% on Windows 7 Ultimate x64 SP1

• include the Steam's install folder at the exceptions list of your antivirus software (like AVG for example)
  
• include the Steam's install folder at the excludes of your firewall software (like ZoneAlarm)
  
• include the Steam's install folder at the exclusions list of your anti-malware software (like Malwarebytes Anti-Malware) {see picture #7}
  
• include the Steam's install folder at the excludes of your maintenance and optimization software (like Ccleaner* and s.o.)

• go to the folder where Steam is installed (for example - C:\Program Files (x86)\Steam) and delete the folder named "appcache"
  
• restart the computer

• click with the right mouse button on "My Computer" and then click on "Properties" . now click on "System protection" and go to the "Advanced" tab . click on the button named "Settings" under the "Performance" section and go to the "Data Execution Prevention" tab . click on "Turn on DEP for essential Windows programs and services only" and then click "Apply" and "Ok" .

• click on "Start" and go to "All programs" >> "Accessories" . now click with the right mouse button on "Command Prompt" and click on "Run as Administrator" (see picture #1) . then add and run these commands (without the quotes) one after another , while hitting Enter after each : "bcdedit /deletevalue nointegritychecks" , "bcdedit /deletevalue loadoptions" , "bcdedit /debug off" , "bcdedit /deletevalue nx" . there's no matter if an error (like this one for example : "An error occurred while attempting to delete the specified data element. Element not found.") appear at the command prompt after some of the commands . just continue with the next commands and steps in the row .
  
• while the command prompt is still opened , run this command (with the quotes) : "the-folder\where-Steam\is-installed\bin\SteamService.exe" /repair (for example - "C:\Program Files (x86)\Steam\bin\SteamService.exe" /repair)
  
• restart the computer

• add "-tcp -clearbeta" at the Steam's shortcut target options (see picture #5) . then click on "Advanced" and place mark on "Run as administrator"
  
• start Steam and go to the "Library" . click with the right mouse button on "Counter-Strike: Global Offensive" , then click on "Properties" and at the "Local files" tab click on "verify integrity of game cache"
  
• now go to the "General" tab and click on "set launch options" . then add this at the blank field : +clientport 27006 -novid -high -threads 4
  
• then go to the "Updates" tab and click on "High Priority - Always auto-update this game before others" . then at the bottom , place mark on "Enable Steam Cloud synchronization for Counter-Strike: Global Offensive"

1. disable active and system monitoring (see picture #4)
   
2. go trought steps 3 , 4 and 5-(the "verify integrity of game cache" part only) everytime before starting Steam

Originally posted by StreamBG:

The ones of you who still encounter the problem , have software that conflicts with the VAC system security . I also had that error and i also was searching for a solution . After trying all of the above steps in that order , my problem was solved . That's why i wrote this guide . So again , if you still encounter the problem , there's software that conflicts with the VAC system security . There's no other logic .

• click on "Start" and search for "powershell" (without the quotes) . now click with the right mouse button on "Windows PowerShell" and click on "Run as Administrator" (see picture #6) . then copy this command (without the quotes) : "Get-WmiObject -Class Win32_Product | Select-Object -Property Name, Vendor, InstallDate, Language, Version > C:\installed-software.txt" . to paste it , simply click with your right mouse button somewhere in the form . run the command and wait some seconds for the list to be created . then go to drive "C:" in "My Computer" , where you will find the .txt file containing the information .
  
• to share the info from the file , use tools like pastebin.com , paste.ee and clipboard.space for example