Computer speed is very slow

HI!
I have brought my new PC. Its RAM is of 3GB and harddisk is of 320GB but still my computer speed is very slow. I regularly defragment my drives. Use C-cleaner to delete garbage.
I also have AVG software(Antivirus) which I update regularly. But still my computer's speed is not improving.
Can anybody guide what other things can I do to make my computer speed fast?
