Just use the Advanced Search (upper right on this page). This allows you to search the entire board, or just a specific forum, like Main or Pictures. You can put in a limit on the last post date, such as last week, last month etc. Although this will still show a 3 year old post that someone just commented on, it will vastly cut down on older stuff.
The other thing I like about Advanced Search, is you can just search for a word that appears in the title of the thread, so you won't get a thread returned with someone's random comment that uses that word.