Open a command line from Windows 7 DVD or Repair CD using this method:
DISKPART : At PC Startup
Then type:
DISKPART
LIST DISK
SEL DISK 0 (replace 0 with XP HD #)
LIST PART
SELECT PART 1 (replace 1 with XP partition #)
ACTIVE
DETAIL PART (get XP drive letter)
EXIT
After exiting DISKPART, type:
bootsect /nt52 c: (replace c: with the XP drive letter)
press enter, reboot into XP
In XP Disk Mgmt, you can delete Windows 7.