If* it is a virus, then you should try to clean it instead of quarantine it since winlogon is needed to logon =]
Try going to Computer > Hard Drive Letter (C:\) > Windows > System32 > find winlogon (winlogon.exe) and right click and scan with AVG > Clean if that doesn't work try using a malware remover, more then likely it's reporting false, depending on when you installed Windows 8 Customer preview, and I'm not sure AVG supports Windows 8 yet I believe Kaspersky has a beta out (2013) for Windows 8 (90 days) but I could be mistaken. If you find it to be clean, just hit Ignore or Safelist. =]
Also try a google search or a Toms hardware search for (winlogon.exe) You will find useful information on it.