Keygens, downloaded from web or P2P, are very often fakes or bundled with some malware - so they are infected.
Also, keygens are often packed with very strange executable packers - so even if the particular file is not infected, it’s often detected because of the packer. Yes, you may call it a false alarm in that case - but I think it’s not very surprising that AV companies don’t really care much about such kind of false positives…
Your USB has no virus, the problem is about the outrun.exe which has some codes that are not virus but avast detect as so.
Go to “computer” then when you see the name of your usb right click on it ans select “Explore” or “Open”.
Don’t know what OS you’re using but with XP and Vista it’s wise to disable autorun on all drives. With Windows 7 it’s disabled by default I believe. Just doing this reduces the chances that the virus will run and then spread but either way you should scan that USB stick with another AV to double check. I wouldn’t trust it personally.