From what I can tell Lightroom seems to slowly uses up memory until it can no longer function properly, I suspect it is not freeing all the memory it allocates. Beta 4.1 had much worse issues but they are still there in V1.0.
I am trying to import my image library (about 300GB of images) into a new database and I have to do it one folder at a time or it just gets part way through and crashes. Even doing it one folder at a time I have to shut it down regularly to force it to release all the memory it has allocated or it will eventually crash.
Christopher, are you aware of the problems related to running Windows XP with 4GB of RAM? Windows can only address 4GB RAM in total, including video card RAM and memory set aside for other devices and system use. Whenever I upped my RAM to 4GB I would get random crashes on a generally unstable system. I'm certainly no expert on this subject but from what I have read it can certainly cause problems.