this is my computer's system, I'm trying to use Blender 2.78, but it works so slow. Is there anything I can do about it?
Keep your expectations within reason and consider how much you are willing to commit to a hardware upgrade should you do so. Computer parts aren't necessarily cheap, so be sure to weigh the costa vs. benefits of a potential upgrade before diving headfirst into this. That said, here is some information about upgrading hardware.
3GB of RAM is limited with today's memory-hungry applications. Upgrading to 8GB or even 16GB won't magically turn your PC into a render powerhouse, but it is definitely a start. Almost all RAM sticks are fairly simple to swap out, and shouldn't require too much technical know-how. However it is essential that you choose the correct type of RAM stick, else it will not fit onto your motherboard. If you're really unsure, take the PC to a local shop. Other upgrades, such as a CPU, are beyond the scope of this answer.
As for other tips to help performance, quit all nonessential applications when running Blender, and keep your CPU from running hot by ensuring the computer has substantial airflow i.e, keep it in an open area at room temperature and make sure the fans are operational.
Best of luck.