Dropbox has announced that it plans to stop supporting Windows XP this summer. The support will be dropped in a phased fashion; on June 26 the Windows XP desktop app will no longer be available for download on dropbox.com and you won't be able to sign into or create an account after this date on Windows XP. If you are logged in, however, you've got until August 29 before Dropbox will automatically log you out.
Dropbox's reason for ending support is that Windows XP is simply too old. Aside from the fact Windows XP support was dropped two years ago, Dropbox says that new versions of its software will get additional features, better performance, and enhanced security, but to deliver these things, users need a newer operating system than Windows XP.
If you don't take heed of Dropbox's warning to upgrade now, you will not lose your files stored on Dropbox when you've been forcibly signed out of your account on August 29. You just won't be able to sign into the app again until you have switched to a supported version of Windows. Of course, you could still log in to your account from the Dropbox website, but that won't continually backup your files.
During the first quarter of 2016, Windows XP still made up around 11% of total operating system market share figures. If you have a copy of Windows 7 lying around, now might be a good time to install it before the free upgrade from Windows 7/8.1 to Windows 10 runs out this July.