That doesn't make sense!
an X86 family 15 is an intel P4, but later on you say AMD 64 Athlon, which is a totally different brand of CPU.
Anyhow, you have enough RAM, which was the important bit, if it's still running the same install of XP as it was 6 years ago then your best bet is going to be back up all your personal files and wipe the computer and do a fresh install of XP, a 6 year old (or even a 2 year old!) install of windows would have picked so much crap up over the years that it will slow the system right down.
It's possible to clean the system up, but to be honest it would probably take longer than a fresh install and you'll never be 100% sure you've removed all the rubbish.