McAfee VirusScan: procedural question

I’ve had McAfee VirusScan on my PC for about 16 months with no problems to date. Every few days, the McAfee Security Center tells me I need to download a new virus definition, so I do it.

Once downloaded, am I now protected from new viruses–or must I run a new scan–even if I haven’t had any virus problems in over 16 months and even if I have been conscientiously downloading the new definitions every time I’m told to?

Problem is, running a new, complete scan takes about 45 minutes.

ALSO: Should I do the quick “heuristic” scan that takes about 10 seconds, or should I scan every single file, subfolder, and compressed file–basically everything, each and every time?

Thanks a lot, in advance.