How to fix windows xp’s slowdown?

I use windows XP service pack 3 all the time. But it does last long. It becomes slower day by day. I use Avira free antivirus to protect my PC from viruses. It does not give so much memory pressure on the ram. After passing several day form installing the XP, every program becomes slower.
They take so much time to start. I use System mechanic and other PC speed booster based software. But I get no result. I do not know what is the real problem? I have changed my hard disk and Ram. But it is as same as the past. It still takes loge time to start any program. My system configurations are below:
Intel Pentium (4) 2.26 Hz, 512 DDR ram, 160 id HDD, Gigabyte 81845GE-RZ (-c)
Is that configurations are OK for windows XP service pack 3? If that is okay, then why I am facing this slowing problem?
I really want to get rid of this problem.
If any body solved this problem,
Please help?
