guy dont panic so much. panic will cause you to do more crap.
first thing to try is to disconnect the power cord, then open the computer and locate the cmos battery. its most likely a small disc shaped 3V lithium cell. remove it and wait 5 mins. if you have your motherboard manual it could help you locate a bios reset jumper but that isnt necessary, just take out the battery for 5 mins.
the default bios is hard coded into a rom chip, its most likely a square chip that fits into a socket so that cannot be damaged from the flashing the bios. when u flash it actually writes new bios to a new location i think. it dont overwrite the old one.
after 5 mins put back in the battery and connect back the power cord. try to boot and see what happens.
if that still dont work then you could call the company and order a new bios chip which would cost about $5.
if however you buy a new board then you dont need to reinstall windows, especially if you buy similar board. if you buy another board and xp dont boot then hold down F8 while it starting up and try to boot up in safemode. if it works then go in device manager and uninstall the IDE controller then reboot normally. the next thing you could try is borrowing an xp cd from a friend, booting your pc in repair mode then go to your system32 folder and look for your hal*.dll file. most likely it would be halacpi.dll find that file on the xp cd and overwrite the one on your hard drive with it. that too should work.
if you have access to an xp cd, there is something else you could do that many do not know. boot from the cd as usual, act like u gona install xp over your existing copy, do not format the drive. it will eventually tell u that setup found another version of xp already installed and ask you if you want to repair it. do that and your fixed.
most importantly if you paid for a legal version of xp then you are suposed to have a cd. sometimes the company you buy it from dont give you a cd in which case you go back and ask them nicely for it, if they refuse to give you or sell you for a small fee, then go get an AK47 assault rifle and return to the store asking for your cd once more.