iam in need of a new OS so Iam looking on to Linux i want a usb bootable
Os for basic computing and web surfing and the main thing is the ease of use i have seen various linux OS but very tough to use i dont understand a thing so if there is very easy and small linux OS it will be very helpful.
Currently Iam using Windows XP sp2

Easy and small is very relative term. And you have not said what distributions you have tried so far. Personally I would say Ubuntu or Linux Mint are the distributions you should be looking at. Even Fedora is pretty nice and easy. And you can create a persistent usb boot for all. That is not a problem.

Puppy Linux Community - Home : http://puppylinux.org

How is Puppy Different?

Small size, ~100MB! This lends itself to some very useful and unique features.
'Live' booting from CDs, DVDs, USB flash drives, and other portable media.
Runs from RAM, making it unusually fast even in old PCs and in netbooks with solid state storage media.
Very low minimum system requirements.
Boot time is well under a minute, 30-40 seconds in most systems.
Includes a wide range of applications: wordprocessors, spreadsheets, internet browsers, games, image editors and many utilities. Extra software in the form of dotpets. There is a GUI Puppy Software Installer included.
Puppy is easy to use and little technical knowledge is assumed. Most hardware is automatically detected.

If you know nothing about Linux, http://linux.about.com/ is a good starting point.

I've tried nearly 20 distros over the years and for simple installation and maintenance you won't go far wrong with Mint. Fedora and Ubuntu are nearly as simple. They will all run well if your CPU has a speed of at least 750MHz and you have at least 512MB of memory.

For anything smaller or slower, try CrunchBang or Puppy.