If you have it in the suspect, which I'm not sure you have the original unencrypted version, as you said you copied it from the chest.
You should open the chest (right click the avast ' a ' icon) and select the Infected Files section. Find the file and right click on it and select export, select the suspect folder as the destination. Since this is a folder you created I would have though you would have permissions in that folder.
Unfortunately as I said I have avoided the lovely Vista so I have no practical experience of it. Have you tried logging on as 'the administrator' assuming you can ?
Will it let you burn it to CD or copy to a USB drive, you may need to temporarily exclude these drives from scans so avast doesn't alert. Once you have it off-line you could transfer it to the XP machine.