Never mind about improving performance yet, get some security software installed as soon as possible -- very unwise not to have such software installed. In my opinion, Norton Internet Security is unbeatable - I wouldn't use anything else.
As for performance, 512MB RAM is nowhere near enough for Windows Vista, no wonder it's sluggish. For decent performance you need to increase that to 2GB at least, 3GB is even better. The low amount of installed RAM is the single biggest cause of your computer's poor performance.