I'm running H&H on several computers, and get different problems on them.
the latest one happens on a rather older laptop with 1.3GHz processor and 500MB RAM, running Win XP SP 3, grafix and sound on-board. java and drivers are up-to-date. the problem affects the autohaven.jnlp client, the gilbertus client and the arksu client (both custom clients up-to-date). the crash happens regardless of running the browser, the Miranda IM client or µTorrent in the background or killing them all before launching any client.
crash details: all clients launch fine, login works, character selection works, game starts fine. then, about 30 minutes after starting, the game screen freezes or goes completely white. the mouse isn't frozen.
process explorer shows that CPU usage by java.exe drops from 80-90% to 0% at that moment. both java.exe and explorer.exe are shown with the window status "not responding". sometimes the client will unfreeze a second or two later, skipped animations (e.g. jumping fish, swinging the axe on a tree) will be fast-forwarded and the game will go on. process explorer shows CPU usage by java.exe back to 80-90%, and both java.exe and explorer.exe go back to "running" status.
sometimes (more often, sadly) the client will not unfreeze anymore, staying in the "not responding" status for 10-15 minutes and longer. at that point I usually kill java.exe and relaunch the client. there is no data loss, no rollback, no items lost from inventory, not even breaking combat - yesterday the crash occured as I tried to shoot a boar. on login I was back in my boat and the boar was still aggro'ed on me.
there are no error messages, too, so I'm lost for ideas except the obvious blaming of obsolete hardware.