I want to install linux on my desktop which already has xp and my RAM is 256MB so plz tell me which smaller version to install to learn basics of linux..is it slackware

I recommend Puppy Linux. It is designed specifically for beginner users with older hardware like yours. Also it runs entirely from CD-ROM so you don't need to "install" it to your computer--it will not replace or modify your existing Windows XP. Puppy will give you a fun and easy introduction to Linux.

http://puppylinux.org

Maybe once you learn more you learn the basics, you will branch out into other distributions or "distros" of Linux:

http://distrowatch.com

You can use Slackware with XFCE or one of the smaller window-managers on that machine, in fact you should be able to use any distribution, if you choose a lightweight desktop environment, like XFCE, LXDE or Enlightenment. Alternatively you can choose to use a window-manager instead of a desktop environment, like IceWM, FVWM, JWM, one of the *boxes or a tiling WM.
My choices would be Slackware (XFCE or a WM), Debian (XFCE, LXDE or a WM), Xubuntu or Lubuntu (XFCE or LXDE), Vector (XFCE or a WM) and antiX (IceWM or Fluxbox).
There are more, this were just a few that came into my mind first.

EDIT: I will not recommend Puppy Linux here to learn Linux. It is not a multi-user distribution and therefore you always have to run it as root. This way you will not learn enough about user permissions, and also are permanently in danger to damage the installed system.

Good point. It has been a while since I used Puppy so I forgot that fact. More info on Puppy's security recommendations:

http://puppylinux.org/wikka/security

There is also plenty of info on user accounts in the Puppy Forums, for example:

http://www.murga-linux.com/puppy/viewtopic.php?t=47410

SliTaz is another "Live" distro for older hardware but has built-in multiuser support:

http://slitaz.org

i would also look into anti-x which is a mepis based distro. it's the easiest and best bang for your buck i've found.

I vote for
a) knoppix,
b) AntiX and then..
c) tinycorelinux.
And stay with the last one. My old box (see my blog) like it. And I am still working in extending it after 1 year of linux..;-)