You should use the corresponding mask, i.e. if you want to exclude C:\folder, you have to enter
C:\folder\*
What Tech was trying to say is that the exclusion mask is directly matched against the filename being accessed - so, if the particular program accesses a file using a short filename, then the short filename has to be in the exclusion list (for the file being really excluded); if the program uses a long filename, the long filename has to be in the exclusions.
Yes, you can exclude the whole drive (x:\*). Or, you can open the On-Access Protection console (leftclick on the avast! tray icon), click "Details..." if not already expanded, select the Standard Shield provider and watch the "Last scanned" field when the database is being accessed. You should be able to see what is being scanned (if the path is too long and is truncated in the window, you can use a tooltip) - and what possibly needs to get excluded.