Using two operating system in your PC.

Open command prompt or type cmd.exe in run. Then type this-
bcdedit /create (ntldr) /d "Windows XP" and press enter.

Then set the path where you have installed your windows xp. For example, D directory. Type following-
bcdedit /set {ntldr} device partition=D: and press enter.

Then type following-
bcdedit /set {ntldr} path \ntldr and press enter.

Now you should add this to boot up screen-
bcdedit /displayorder {ntldr} /addlast and press enter.

Now reboot your computer. Hope it is done.