Now I'm using windows xp,but i just planned for ubuntu.Configeration of my pc is pentium 4 with 1gb of RAM.Is it good for dual boot? Another thing is now I'm not able to play any hd Videos.After installing ubuntu can i able to play all this stuff?
Yes, your PC is more than able to dual boot. Perhaps give a thought to Xubuntu, although I don't think that's necessary.
As for your second question -- HD is demanding. The fact that all newer PCs are able to effortlessly play it does not make it any less astoundingly resource intensive. Given your hardware, I estimate it's going to be difficult to get HD playing fluently. But -- that's no reason for not trying :)