I'm afraid it's not possible.
If you move the (complete) content of the <avast4>\Data\Chest folder into the same folder on a different computer (i.e. into another avast! installation), you should be able to extract the file. Or, possibly the virus guys may be able to help you, if you uploaded this folder on our FTP - but it's not possible to access the virus chest without windows.
Btw, what version of avast! do you have installed? Any newer version shouldn't allow you to move this file, no matter if anything is detected inside or not.