You can disable McAfee VirusScan easily enough.
Double-click the taskbar icon to open SecurityCenter
Click Virus and Spyware Protection
Click Real-Time Scanning
Click Turn off and tell it for how long.
No antivirus is 100% guaranteed but never, ever install more than 1 on your machine or you may just open the door to infection because they will clash with each other.
Instead use a good anti-malware tool such as Malwarebytes or similar...I mentioned a few halfway down this page under the title '3rd Party Tools': https://community.mcafee.com/docs/DOC-2168
Ask any independent security forum and as I was a Microsoft MVP in Security Awareness for a while I should know.
Rule of thumb: Have only 1 each of antivirus and software firewall installed. Extra anti-mal/spyware tools and a hardware firewall as in those found in routers are fine. But research software clashes, some are good, some are terrible.
The best defense is to avoid risky websites, downloads and file sharing and to keep Windows and all your software totally up to date.
People will often, for example, not update Internet Explorer, ever, as they use Firefox or another brand...big mistake. There's a security hole right there, whether or not you use it, as some applications do use it, McAfee included.
I trust that this is Vista SP2? If not it is unsupported so make sure you bring it up to date. Disabling antivirus during a service pack installation is a good idea.