Try opening Reactor as admin, if that doesn't work run CCleaner and Adv System Care and try again opening as admin .
If you do all this and it doesn't work im sorry but you need to download the game via Page mirrors.
Or.. you can follow these steps to download it without Steam, in 4 easy steps.
Written by /me
1. Go to ijji.com, when there you should see their list of games Soldier Front, Gunz etc, AVA should be there, click it.
2.Now that you've clicked on it at the top of the page you should see the Download button, its next to Add G Coins.
3.Now you should be on a page that list's IJJI's games and their Sizes and download server.From there scroll down and click AVA, select your server and then once you've select the server it will lead you to another page.
4.Now your on the download file page, just click Download and now after your done your ready to play some AVA.
start>control panel>programs>uninstall a program>uninstall, and re-install. I think that will fix your problem, give it a try.
On the reactor click "Repair".