AVG Free automatically scans MDB files when you open them, and this cannot be turned off in the Free edition. Even on a fast PC, it could take 20-30 seconds depending on the size of the database.
This presents itself in two ways:
1. You are using Access 2000-2003, just opened the db moments ago, and you want to do a quick compact & repair. Access tells you either "Cannot open database '(database name)'. It may not be a database that your application recognizes, or the file may be corrupt." or "Cannot open 'database name', file already in use."
If you wait a little while and try it again, the compact & repair succeeds.
2. You are using Access 2007 and wish to convert a 2000-2002 format MDB into the new ACCDB format. Access tells you "You attempted to open a database that is already opened exclusively by user 'Admin' on machine '(machine name)'. Try again when the database is available."
In this case, waiting does not work. You need to close, and then re-open the database while holding the Shift key, to bypass any AutoExec or startup code. Then, you wait for little while and repeat the Convert and it will succeed.