Things you could try
1 check that you do not have a disk in the CD/DVD drive. Blank/RW CD/DVD disks can prevent the computer from booting due to the CD/DVD drive being set to boot first.
2 Try unplugging the power lead and hold the front start button in for at least 30 seconds, plug back in and try again.
3 If you have the XP disk not the recovery disk you could try repairing windows.