Try a repair installation first. This may allow you to boot properly. Also
notice that before you ever connect to the Internet again, you should enable
your internal firewall or your chances of being infected with some
worm/trojan increases to just over 90%.
Format/Delete all partitions and perform a Clean Install from your Windows
XP CD.
(When you boot with the CD, after setting this option in your system BIOS,
there will be a part where you can choose which partition to install XP
onto. At this point, if nothing is working, you can delete the partition
information and create new partitions to install XP onto. This *will*erase
everything without exception.)