How to use windows seven after installing window xp ?

I am assuming you mean that you have one HDD in the computer with multiple partitions, the apps maybe there in the drive, but you have a big problem as the XP install has modified the MBR and destroyed the boot information for Win7, normally if XP is on the drive and if you have partitioned the drive into at least two Partitions then you can insert the Win7 disk, choose custom installation and install it in a partition that has no OS in it, but if the computer has Win 7 and you try to install XP, Win 7 is crippled and I do not know of any way to undo what you have done, there is a thread here of some that has done the same thing as you and they haven't found any way to undo the damage, sorry-
so the only way that you are going to repair this is to wipe the drive, do not bother to format it, it will not remove the file in the MBR that needs to be clear to do a install, use The Ultimate Boot CD(it's a free download,type UBCD in your search, do not use ubcd4win), burn to a disk, boot from this disk, select HDD tools and wipe, it's the only way

Persianmajid, please do not erase your data.

Please check this tutorial, it explains how to fix your boot problem. Your case is the second method, when you had Windows 7 installed first and then installed XP.

http://www.sevenforums.com/tutorials/8057-dual-boot-ins...

I am reading this article but I don't see how to repir an overwritten MBR, but it won't help him, he already destroyed the ntldr.dll in the MBR, there's no way to repair his problem

Number 13, I usually do not like to openly disagree with another poster, since you are here just trying to help somebody without getting anything in return but the satisfaction of helping. But when you tell the Op to delete everything he has I have to correct you.

You are not correct. He indeed ruined the Windows 7 boot configuration data (BCD) when installing XP after that, but it can be repaired with EasyBCD, if the steps in the tutorial are followed (or BCDEdit if you don't want to use a third party software, but that is a command line program and more difficult to use). The problem is not with ntldr (which is not a dll) but because XP has now written a boot.ini that won't allow Windows 7 to load.

Even with a damaged MBR wiping out your drive is not the only solution. There are repairs that can be attempted, please check http://www.ntfs.com/mbr-damaged.htm
But then again, it is not the case here, it is just a matter of repairing Windows 7 boot loader and then adding XP loader as a boot option.

Again, sorry for correcting you on the thread, but I was afraid your solution would cause him unnecessary damage (and possibly irreparable, since he doesn't seem to have backups either)

cool, ok, hats off to ya, any repair that saves time, and reduces data loss I like, I have copied your comments and thank you

Thank you Number13!

There is no need to wipe the drive.
Install Easy bcd onto the XP system and let it find the W7 install.