oh, trust me, once you get foobar set up the way you want it, you'll never go back. it offers a lot more flexibility than winamp.
for a letter filter, use this:
$left($swapprefix(%album artist%),1)
the $swapprefix bit move (the, a) to the end so that "the cure" is filed under "c" as "cure, the". might also want to use that in your artist filter. the rest just grabs the left-most character of what remains.
if you don't want the prefixes swapped, then just use:
$left(%album artist%,1)
i can also help you set up a custom script to swap prefixes from the Norwegian language if you want
if you like accessing your library a lot of different ways, you might consider skipping the filters component and using the Library Tree component instead. it offers a tree view with three primary nodes - library, autoplaylists, and playlists. within the library node, you can set up any number of custom nodes, with as many levels as you like. for example, you an make one "letter - artist - year - album - track", or "decade - year - artist - album - track"
of course, you can do this with filters too, but you end up getting a lot of tabs which can clutter the UI.
and by the way - foobar supports multivalue tags. so you can throw as many entries as you like in %genre%, and the track will show up under all of them. you just have to tell it to split multivalue entries manually, that's the only catch. in the properties dialog, select the field, right-click, choose "split values", and enter go from there.
you can also use whatever custom tags you want. anything at all. i have a %style% tag which is basically a sub-genre. so in the "genre" tab of my filters pane, there are three filters: genre, style, and artist.
i also have custom tags like %skip% which i use to tag things like non-music tracks, single-track live bootlegs, blank tracks, interviews, and other things i don't want to hear when i throw things on shuffle. i use it with the foo_skip component.
another tag is %live%, which just gives me a simple visual marker in my playlist for all live albums/tracks.
yet another is %country%, which is responsible for the flag images you see in my screenshot.
foobar can do just about anything