hi Bagpuss, you're more than likely just trying to run too many programs at one time or else one of your running programs has temporarily become corrupt. Just reboot your machine and everything should be fine. If the message continues to appear after that, you may need to take further action.
256 mb (this is Ram, you can add more) is not a lot of memory in this modern age. Software writers are increasingly adding more bells and whistles to their programs and in turn computers have had to become more powerful in order to run them properly. Most computers on sale these days come with a minimum od 1gb of memory, thats 4 times as much as you have, so it might be an idea to install some more. how would i do this ??
In the meantime, your computer sounds to me like its struggling to cope with whatever programs you're trying to run. A temporary fix may just be to cut down on the number of things working in the background on your mmachine. Things like virus checkers, adware checkers, etc all soak up pc resources but they are a necessary requirement, however there are lots of other programs that can continue running in the background when you dont want them to, some of them can be malicious or at least unwanted. You should ask a knowledgeable friend to check your computer when you can, this just a suggestion, no need to worry about it.
Your other details look good:
34 gb is the size of your hard drive and you've still got 23 gb free, thats oodles of space, incidentally this is where all your programs, files, photos, documents, etc are stored. Its completely seperate from memory, think of it more in real estate, ie, a place to put something. It stays there permanently. Ram, on the other hand is short term memory, it only holds stuff temporarily, once you switch your machine off, everything in ram is removed.
Virtual memory is a small amount of your hard drive which has been allocated by your computer to assist the ram. Its a bit difficult to explain here without going into detail, suffice to say that its normally best to just use the automatic settings. No need to bother with this.