Sorry to say,
officially Windows XP is no longer available for download as a trial and moreover, it can not be bought from Microsoft's official store. It is so, just because currently Microsoft has ended it's support partially to Windows XP (SP1 & SP2) and Vista (SP1) and will fully end it's support in the Mid of 2014.To know more, visit, http://windows.microsoft.com/en-US/windows/help/end-support
.If still you are willing to have Windows XP, you have to download it online (which will, of course, be illegal) from torrent sites or something like that.But if you love to buy a copy online, you should visit: http://erclk.about.com/?zi=5/2rbm
Before you proceed, you may like to read this article: Where can I download windows XP?
.Hope this helps, Mrityunjay.