serious virus

ok, ill start by saying that i did not do anything to get this virus, it happened when i was away from the computer (hacked? i dont know). i had on AVG firewall and everything, so i do not know how this could have happened. it occurred late last night (sunday).

i get back to my computer and it says that the hard drive is missing, then it reboots and i get as far as just before typing in my password to log in (i use windows XP). after recieving the same message there, i reboot again, and this time it will not even load windows. ever since, i have not been able to get past boot up. the message i get is basically that there is an "unknown driver" or it does not even exist. what is fishy is that it allowed it to boot windows at first even though it rejected me before i was about to log in (boot vector virus? or not boot vector virus? if not, where would it be?). Another clue as to weather it is a virus as apposed to my hard drive is that when i use the windows XP operating system CD to go to setup, it says my hard drive is there. i am allowed to go into the "repair windows" option on the CD, but the options seem like they can screw up my system easily if i choose the wrong command. so i left them alone. i could install windows, but i would have to replace my previous installation, and it also says "my documents" could be deleted (i do not want that). if it comes down to it, i will replace it, but that is why i am here. maybe it is easy to fix, or not. i cannot give a HJT log because i obviously cannot get into windows. i am sending this via another computer nearby. if i could just access windows, i would be able to do a combo fix/AVG antivirus scan to get rid of it. i have never experienced a boot vector virus, or anything that runs outside of windows, that is why i need your help. any would be appreciated, thanks .

P.S. i tried the computer again today, it got to my log in screen (i was pretty happy), but as it was logging in it froze. i turned it off because i feared the worst (hacked)

 

anyone?