Windows in its infinite wisdom protects files in use (even malware) or in system folders, so it is likely that avast! can’t delete or move files in use. So schedule boot-time scan in avast’s menu if you have XP, win2k or NT, otherwise boot into safe mode and run an avast scan. This should ensure that the file isn’t in use and avast should be able to deal with it.

To enable a boot time scan. Right click the avast icon, select Start avast! Antivirus, Menu, ‘Schedule boot-time scan…’

You should also consider cleaning your Temporary Internet Files.

Two handy tools, ClearProg - Temp File Cleaner or CCleaner - Temp File Cleaner, etc.