For configuration data, XML is good because it lets you store data in a human readable and modifiable format with just a text editor it is also easily transferable via a file unlike registry settings. I find it takes more to work with then just ini files though if your just going to store simple value pair values. If you have more complicated structure to the data your storing then XML is defnitly the way to go.
The only reason why you would want to use an access database over XML is if you have some very complicated data manipulation that you would like to do with SQL quieries. You can do quite a bit with just XML and a reader that supports XPath quieries though, so I doubt you would want to introduce the overhead of using access.