You should *never* use spaces in file or directory names on the Internet.

The use of spaces in file and directory names is strictly a Windows convention -- perfectly permissible (and almost encouraged) on desktop boxes running the Windows operating system.  It is also permitted and perfectly functional in Microsoft browsers.

The Internet in general and most servers in specific, however, remain devoted to UNIX-based naming conventions.

Aside from some servers still having difficulty with this, older browsers (notably Netscape 4.x and prior) will see a space in a URL as the end of the URL, and truncate (ignore) everything after the space.  For instance, this URL

http://www.somewhere.com/my directory/some page.html

would be seen as

http://www.somewhere.com/my

with the usual result being that older browsers will wind up reporting a Bad Request or a 404 - Not Found.

The usual method of correcting this is to replace the spaces with _ underscores, like this:

http://www.somewhere.com/my_directory/some_page.html

While it is indeed true that Microsoft Internet Explorer and newer Netscape (and some other non-Microsoft) browsers will replace the spaces with %20 and proceed properly, there can still be problems with breaking certain JavaScript scripts, some Perl and PHP scripts, and other services, if spaces are used anywhere in a URL.

Until recently, this could also cause problems with (legitimate) search engines, 'bots, and crawlers; and the situation may remain so with some of them.  A more serious nuisance is off-brand private 'bots and "site downloaders" that haven't the sense to properly interpret a space, and fall into endless loops calling non-existent pages on your site thousands of times at relatively high frequency -- often burning enormous bandwidth and potentially causing server problems.

In view of these considerations, if you have spaces in directory or file names, it's wise to correct them as noted above -- sooner rather than later!