Startup priority of programs

I just installed a new virus scanner and want to configure that after logging in as user the virus scanner boots as first instead of other programs like Dropbox.

What's the simplest way to adjust booting priorities?

Apart from 3rd party applications (ex: Set Delay time for Startup programs in Windows 8 / 7), you could use the Task Scheduler.
You can create a task that will trigger at logon, and you can also add a delay.

Here is a good tutorial: http://www.eightforums.com/tutorials/2070-task-scheduler.html
And here is one specific to Windows Defender : http://www.eightforums.com/tutorials/14264-windows-defender-scheduled-scan-set-up-windows-8-a-6.html

AV programs are what's known as "Ring 0" programs, meaning they will be loaded very early in the boot process. Just because you don't see the GUI for the AV program doesn't mean that the scanning engine hasn't been loaded and is actively doing its job. Beyond that, there's really no direct means to control the order programs start in because there is no order. You have your two basic levels of programs on an OS like Windows 8. There's your kernel level programs, which would be things like drivers and the AV scanning engine, then there are your user space programs, which is everything else. The programs you use to configure and control drivers or your AV scanning engine are completely separate programs, so don't make the mistake of confusing them with the scanning engine or driver themselves.

There's no real set order for when things load as far as user space programs go. The process for each and every one is started essentially at the same time in the boot process, so it's just a matter of how long each one takes to start. Something like Dropbox will probably take a significantly lower number of CPU cycles to finish initializing compared to say an AV program like Norton for example.

Thanks for the explanation, Asvent.

The reason I wanted to change the order was because I got viruses when I opened Chrome before my AV program was loaded.
I guess I just can better wait with browsing until my AV program is loaded and shown on my Task Bar?

But I will take a look at the Task Scheduler tutorial later on.