how can i speed up my computer?

I need to speed up my computer and i dont want to mess anything up so please try to explain in a detailed way. I have emachines windows xp i have avg virus control and ad ware i have never defragmented my computer or did registry check thing because i didnt know what it was. My computer works "ok" but it is slower than before please help me of such things as which virus scanner i should get or spyware or how to do defrag etc..

Thank you