I used partition magic to partition my hard drive.After installing redhat 9 winXP won't load up. If I select XP in boot loader grub it just reboots my computer. What do I do.?

Did you back up your XP data? My experience has been that you need to load Linux first and then load XP second.

I have a backup but XP hasn't been erased. how can I fix the boot loader to work with XP.

Try adding this to grub's config file.

title Windows XP
rootnoverify(hd0,0)
makeactive
chainloader +1

Here's a good info source:
http://www.geocities.com/epark/linux...w2k-HOWTO.html

I use dual boot too, but my problem is that after I reinstalled WinXp, my computer boot up in Xp, and Redhat load sequence disappeared just after I performed Xp reinstallation. I tried to solve this problem using a boot diskette but I failed to repair Redhat. I also tried using fdisk and Partition Magic but it was useless.
Is there something I can do in order to reestablish Redhat boot sequence?
(I use Redhat 9.0)

Floreen,
The best bet is to search this site or google for HOWTOs on how to use the NT Loader to boot into either linux or XP.
Try the terms : NT Lilo Boot Howto, or NT Grub Boot Howto.

First you will need to use the rescue system to get back into linux and change the boot device to the linux partition. For lilo, this means changing the boot= entry and executing the lilo command as the root user.

Then the howto will explain how to copy the first 256 bytes to a file on the C:\ drive, and edit the boot.ini file so that you are given a XP boot selection menu when you reboot the computer.