There are various ways speakers can embarrass themselves at conferences by projecting personal information or embarrassing stuff onto a screen in front of many people. Background images featuring non-professional levels of nudity are the most well-known problem, but others include accidentally popping up a personal chat session, interesting titles being shown in your browser history, the contents of your clipboard being more informative than you’d like, that kind of thing. You don’t only need to protect yourself against overly sexual material (well, unless you’re presenting to the BDSM community or something), there’s also password leakage, and just the vague embarrassment of showing professional peers a log of you and your husband discussing whether or not to have pork for dinner.
It occurs to me that with a bit of discipline about files, the easiest way to avoid this on laptops where you can set up a second login is to do just that: have a second account which has suitably bland background, in which you only access presentation related websites, in which you don’t IM and so on. Set the password to something totally unrelated to your usual passwords, and change it immediately after each presentation, just in case the worst happens and you type it in such a way that it appears on the screen.
And then do all presentations logged in as your special cleaned up for the general public laptop user. If you don’t even want cross-presentation pollination you could re-create the account for each presentation or have a script that does so for you, just to be sure that there’s no browser history and such.