Originally Answered: Perv hubby?
OK, first thing to do is run a thorough virus scan, adware scan and spyware scan. Your husband just may be telling the truth in this case: there is adware that will pop up porn sites at random intervals.
Second, don't discount the possibility that it is your 7yo - if it's random and intermittent (that is, it only happens once in a while), it could be nothing more than a mistyped address and an onclose() loop -- some less than reputable sites will pop up a new browser window when you close the current one down.
Now, with the "other views" out of the way, let's assume your hubby is watching sex online, and is smart enough to delete his history.
Assuming you use windows, the easiest way is to go to "C:\Documents and Settings\", and then to the folder that's named whatever the screen name is (or "owner", depending on your settings). Now, let's dig a little deeper... "\Local Settings\Temporary Internet Files\Content.IE5\"
Now, before you go any further, get the kid out of the room. Go into any of these folders. This is the stuff that doesn't get shucked when you do the Tools>Internet Options>Delete Files... Delete All Offline Content routine in IE. Right click, View>Thumbnails; then Right Click again, Arrange Icons By > Type.
Now, unless your hubby uses the Run As... command with his browser, this won't tell you *who* downloaded it, it only shows whats been downloaded.
Then, ask yourself if it is really a big deal is if he's looking at porn; or if he needs to be more discreet about it to ensure that your child isn't exposed to sexual images until you as a couple determine that he/she is mature enough to understand them; or if what's bothering you is that he didn't invite you to watch, too.