Anyone can help me?
I deleted by mistake my antivirus entry on the startup and now the program is not starting nor showing the tray icon unless I open it again each time I start my computer.
Is there any way to fix this issue?

You should be able to reset the autostart through your AV's gui somewhere?

I might be missing something, but I can't find any way to do that in my AV (Avast), and this is the second post in a very short time about deleting stuff from startup with CCleaner.

http://forum.piriform.com/index.php?showtopic=15225

I assumed a System Restore would do the trick, but I'm reliably informed that this isn't the case. That surprised me, as I once again assumed that startup data would be included in "System Volume Information".

That not being the case, maybe a warning in that section of CCleaner to the effect that deleting Startup items with CCleaner is irreversible.

Apologies if that warning is already there, but there's nothing in my startup I can delete to find out.

Back to the problem, maybe it's gonna have to be a case of re-installing your AntiVirus software. If you go down that road, find out if it needs to uninstalled first, or if you can install over it.

If you're not sure about that, tell us which AV you have, and then we can more accurately help you fix it. Maybe it can be done via Humptys suggestion.

Run MSconfig. start up entries should still be there and just put a check on the app to re activate

If there isn't an option within the gui to add to startup then the program needs to be reinstalled.

Indeed, that's the quickest solution and requires no trouble shooting.