If you keep your DATs up to date, the transition to Windows 7 should not matter.
Anti-virus software alone is not enough to keep a machine secure in today's world - it is an unfortunate fact that no Anti-Virus vendor is able to promise 100% detection rates so there will be times when you get hit by something bad. New malware is created and published all the time, therefore keeping your AV software up to date should been seen as just a part of your security defences. McAfee recieves over 50000 potential malware samples on a daily basis, and this number will continue to rapidly climb. As quick as we're writing protection and designing new technologies to fight the wave of malware, the bad guys are working hard to find new ways to beat the system.
It's not all doom and gloom though, with a bit of knowledge you can do a lot to protect yourself, your family and your data from the threats and attacks that are out there on the web.
For more information please see: How did I get infected when I have McAfee software installed? (TS100771) and 13 Ways to Protect Your System. There are also many articles available in McAfee Security Advice Center which are well worth a read.
Hope that helps,