@goldy_connect, I had tried with all the accessibility options. But no help.
@iMav, while typing there is no problem. So, I don't think shift key is the culprit.
@Choto_cheeta, I have tried with another keyboard. But still the problem persists. It happens in the desktop only.
@ravi_9793, system restore is turned off. So, I think, there is no other option left other than installing Xp again...
Actually, this problem is with my friend's system.
I know that it is a feature in XP. In my keyboard(iBall's-iKey), there is a "MARK" button. If I press it once, this feature gets activated. After that whatever I click, all the icons above it get selected. Even if, I click on some text, all that's above the cursor gets selected. To turn this feature off, I have to press the "MARK" button again...
But in his system, there is no such button. So I don't know, how this got activated. I think there is some key combination, which was accidently pressed. Accesibility options doesn't have this setting.