There is no such function, as a) it would somehow require editing the file properties, b) if it could be done it is likely to be time consuming and c) depending on what the file is (with security, digital signature) could change its file size and possible fail validation.
If it can be done, I would put the files in a password protected folder (or if not possible, in a password protected .zip file) on the stick with a text readme.txt file in the root of the flash stick containing the password to be able to access the folder/files in the .zip. Giving an indication that the files had been scanned first.
@ Rednose
The avast virus cleaner is of very limited use (certainly in this case), as it only cleans a small subset of virus infected files, it isn't a general anti-virus scanner.
