How to decrypt in windows xp after installing new windows xp os

You can't do it (not if you used XP encryption rather than some third-party software).

r u sure.???..i wil lose sme very important datas..pls if u get 2 knw sme solution ...pls..remind me..thank u

If it's that important you could try something like this, or a data recovery service may be able to do it for you. I'm not aware of any free (or even cheap) utilities to do this.

You could always download the trial version to check; it won't recover anything but it will tell you whether it can or not.

The crucial thing is not to write a single byte more to your harddrive. Each time you write anything, you risk overwriting data necessary to decrypt the data (if that hasn't happend already).