Lets be clear, avast didn't delete it, it detects it and gives you a number of options and delete is only one and probably the worst. Deletion isn't really a good first option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate. Lesson over.
When avast deletes a file it doesn't send a copy to the recycle bin or the chest (unless you specified that option), it is gone. So you would need a file recovery program that can recover deleted files, provided too long hasn't elapsed the file still exists, just that there is no reference to it in the file table.
Try a search for Recuva as there is a free recovery program and by all accounts may be able to help.
However, I would have to question the wisdom of recovering an infected file (avast would alert again).
What was the file name and location, e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe
Why do you need it now ?
