Hi, this has been happening recently.
I usually try to create a world (usually superflat WITH structures) and I'll get to the "Loading World – Building Terrain" but it usually just sits there. When I tried to make one before, I actually got the message that I DID run out of memory, but the one I'm trying to create now is not responding.

I think this started around 12w37a-12w38a – I know it just started a few weeks ago, before I got 1.4.2pre. If it's something to do with programs, all I usually have open is Google Chrome, Spotify (though I usually close it out) and the Minecraft program itself.

And for this session, all I had open was Minecraft and Google Chrome. Sometimes, if it's really laggy (and I mean REALLY LAGGY), I will close Google Chrome, although it still crashes sometimes.

EDIT: I have 1790MB of RAM. (Random Access Memory)
Processor: AMD Athlon(tm)Processor 2650e, ~1.6GHz

Any help will do!
Alyx