How I can convert vista to xp on sata hardisc ?

You would need a version of XP with sata drivers to install

There are some around but don't know where, that is legal

Hopefully others will add to this to help out

But when you get the sata drivers all situated, steps below:

How to Format Drive and Install Windows from Scratch
Geek to Live: How to format your hard drive and install Windows XP from scratch

Doesn't XP work fine with sata hard drives as-is without drivers? I built my computer myself and I have a sata hdd in it. I didn't have to install any special drivers.

I think the bigger issue here would be making sure that the rest of your hardware would support xp. You could always contact the manufacturer of your PC. They should be able to help you with the downgrade.