These actions take a lot of time and a lot of computational resources as well. Which is also true for watching (highdef) movies. If your PC has the "horsepower" to do all this in the background without making the movie you are watching stutter, then by all means do so.
But I fear that for most PC's the I/O throughput demanded by your request will cause a "less than optimal" viewing experience.
Windows 7 already keeps itself a lot more tidy than previous versions of Windows and by default is scheduled to run the (improved) Windows defrag software.
You could even create a simple batch script that starts the defrag software, the antivirus software and then VLC. When you have the urge to watch a movie, use the batch script and select the movie you want to see. That would be the simplest solution where you yourself are in total control.