The simple way to actually connect to an XLS is through VBScript, but your server will have to support it (which most do).
Another way would be add an ODBC to your servers connectors pointing to your XLS or MDB or whatever Microsoft data file you choose and use JavaScript or any number of languages to connect to it, retrieve data, make changes, etc. The downside is that you must have Administrative rights to the computer hosting the file. You can use a homecomputer to use as server for that file only for instance and link it to your website, but it will affect loading speeds. This will however require you to take a closer look at your home internet security...!!!...but will enable you to make 'live' changes without going to your website. This method is often used for webshops and online catalogues, places where data can change on a daily basis but a database server is undesirable.
The simplest and easiest way is as mentioned before the use of GoogleDocs, but that will require conversion of the document. You won't be using your XLS as datafile. The downside here is that if you make any changes offline (not through the website) the changes will also not be visible, because you'll be editing a completely different file. So you'll need to convert and upload it to GoogleDocs each and every time.
As you can see the method used greatly depends on how often you plan on updating the data, how much traffic you expect and how secure you want it to be.