Are you saying that the computer itself won't start or Windows XP won't start.
Do you get the beep noise and a couple of screens of white text on a black background?
If you do. Press the 'F8' Key after the first screen of text and hopefully you should get a recovery menu for Windows XP. Select 'last known good configuration' and try that.
If that doesn't work, do the same F8 thing and try selecting 'Safe Mode'.
NB Leaving a disk in the floppy drive should not cause the PC to bugger up. Make sure you get the 'full' story from your Dad