If you are talking about C:\Documents and Settings\UserName\Local Settings\Temp\_avast4_ folder, to all intents and purposes it should be empty ?
If it isn't empty and has unpxxxxx.tmp files in there then you have experienced and error during scanning or some other error where the unpacked file remains in the folder. On successful completion of a scan the files are usually deleted.
However, you should be able to delete them, deleting the _avast4 folder itself though could cause a problem.
