Whenever I've experienced this problem in the past, it was because the Internet Guest account (IUSR_MACHINE), which is by default part of the "Everyone" group, does not have Write permissions on the database file (.mdb). To fix this problem, use the Security tab in Explorer to adjust the properties for this file so that the Internet Guest account has the correct permissions. Even though you have full permissions, it is the account that the website is running under that needs the permission.

A second cause of this error is that the database was not opened with the correct MODE for writing. If you perform the Open on the Connection object, you use the Mode property to indicate the permissions on the connection - for example...

SQL = "UPDATE Products Set UnitPrice = 2;"

      Set Conn = Server.CreateObject("ADODB.Connection")

      Conn.Mode = 3      '3 = adModeReadWrite

      Conn.Open "myDSN"

      Conn.Execute(SQL)

      Conn.Close

By default the mode is 0 which generally allows updates, but setting the mode manually often clears up the problem.

Good Luck!