There are various ways it could be done. It can almost be done now.

For example, the user could go to Advanced Search. There is already a list of the 29 sub-categories there, under the heading "Search in Forum(s)." Currently you can select which forums to search by clicking on a forum you want so that it's highlighted, and then clicking on another you want while holding down the Control Key (at least in Windows that's the proper key for this).

So, for example, if I click on only "Devices:N800" and use "gpodder" as my keyword, I get 12 hits, all in threads that are in the N800 category (or, as this website likes to put it, in "Forum: Nokia N800"). If I use the Control key and add the category for "OS2008 / Maemo 4 / Chinook - Diablo" (so that I now have two categories selected) and I again use "gpodder" as my keyword, I get 19 hits (that is, 19 threads). (As you would expect, if I select only "OS2008 / Maemo 4 / Chinook - Diablo", I get 7. If I use the two I already used plus "Maemo 5 / Fremantle," I get 28.)

So having checkboxes, instead of having to hold down the Control Key, to select multiple categories (or what the website refers to as "forums") is trivial. It's just clearer and easier (at least to me).

Then I want the website to remember my last selection of "forums" (what I usually call Categories or Sub-Categories). (Currently the website resets it each time once I close the search.) Once it is remembered, I want to be able to hit the New Posts button and have it do exactly what it already does for New Posts -- but omitting threads according to the filters I had set when I was last in Advanced Search, that is, according to the "forums" I had selected when I last did an Advanced Search.