No, McAfee Security Scan Plus won't fix all the security issues you have щn your computer when Windows is running, but it checks the most vital aspects of the operating system: antivirus information, firewall status, security software. Besides thatб it will also scan applications for known security issues. In case ша you have something related to the items I have mentioned, then yes, McAfee Security Scan Plus will fix them.
For a more accurate scan, I recommend using Secunia PSI: http://secunia.com/vulnerability_scanning/personal/
It can be used for applications that can't be fixed with McAfee Security Scan Plus.