What version of Avast do you have?
I would suggest simply turning your AV off whilst you play Luna, it isn't dangerous. But if you do get a lot of viruses normally, then obviously that isn't a good idea.
What you can do is add Celestia Luna to the Avast exclusions list.
What you should do is this;
1.) Right click the Avast icon in your system tray (bottom right, by the computer clock) and go to "Avast! shields control >" and select "Disable for 1 hour" (Avast will still be running, it will not be switched off, it will just stop auto-scanning for an hour)
2.) Again right click the Avast icon in your system tray and this time select Open Avast! user interface.
3.) In the Avast window, click "Settings" then "Exclusions" then select "Browse" and find the folder where Celestia Luna is installed. The default for Windows user is
C://Program Files/Celestia Luna Online Alpha
4.) Select the folder and click OK to save changes.
Extra--
5.) You can also prevent the Real-Time Shields from blocking the Celestia Luna client by clicking the Real-Time Shields tab on the left side of Avast, clicking "File system shield" and select "Expert Settings". Click "Exclusions" and then "Browse" to the same Celestia Luna folder as before. Click OK to save.
6.) Go back to the right click on Avast in the system tray, go to "Avast! shields control >" and Enable all shields again.
That should stop Avast from scanning your Celestia folder when you try to run the game.
I would also advise you to run Celestia Luna as the admin when you play, by right clicking the Celestia Icon on you're desktop and selecting "Run as Administrator".
Hope that helps you solve your problem, let me know if anything goes wrong!.
Note: This might be different with older versions of Avast.
- Wist