If you have dual boot system and you delete the first partition, you also delete the boot files needed to load the system: boot.ini, NTLDR, and NTDetect.com.
You will get a NTLoader is missing error message during OS boot.
You should be able to boot to the XP installation CD, get into the Recovery Console, and then run some commands. At the “Welcome to Setup” screen, press F10 or press R to repair, and start the Windows Recovery Console, and type the following commands:
FIXMBR C:
FIXBOOT C:
COPY CDDrive:\I386\NTLDR C:\
COPY CDDrive:\I386\NTDETECT.COM C:\
BOOTCFG /Rebuild
(Where, of course, CDDrive is the letter of your CD-ROM drive.)
Then reboot by typing the EXIT command.
NTLoader error should be gone now.