You have installed Linux first on your harddrive, but you feel yearn with Windows XP (you must have the original product). Then you install the Windows XP immediately. You kill your time with Windows XP for long time, then you feel yearn with you Linux (PCLinuxOS). Nah..! You cannot boot to Windows anymore. Why? And what should I do?

Don't panic! You just need reinstalling GRUB to your MBR. Follow these instructions..

In this case, I use PCLinuxOS, and GRUB Boot Manager

Boot your computer with PCLinuxOS Live CD

Get into console

1. Type "grub" , then you get into GRUB environment
    grub>

2. Type find /boot/grub/stage2
     like this : grub>find /boot/grub/stage2
    Then you'll get the location you've installed GRUB/Linux before
      (hd0,0)    -> your linux was installed on first partition

3. Type root (hd0,0)    -> because your linux was installed on first partition
    grub>root (hd0,0)

4. Type setup (hd0)
    grub>setup (hd0)

Done! Reboot your computer now.. You can get into Your Linux / PCLinuxOS again.
Wait... and how about Windows XP? I have paid my license and now I cannot use it. Be patient.. please.. :) Please don't be angry..

We just need to add a few line of code in file /boot/grub/menu.lst

Add like this into your menu.lst
At this example, your Windows XP was installed on third partition, second partition as extention partition.

title My Old Friend XP
rootnoverify (hd0,2)
makeactive
chainloader +1

Save you menu.lst
Reboot your computer again.

Form now on, you can get into your PCLinuxOS or Windows XP again, anytime you like. There is menu to choose them, and they're worked! You are the One to choose which weapon you like, Linux or Windows.