BSOD : session5_initialization_failed

[kirby_blue]

Hi all,

I have :
Win XP SP2
Avast Home 4.8
Latest virus definitions

My problem :
I caught a virus (though avast couldn't detect the exe file as a virus)
Upon rebooting (I still wasn't aware that I was infected), avast told me it had detected a rootkit and asked me to plan a scan on reboot. I did so.
Now, whenever Windows boot, I get a blue screen with message : session5_initialization_failed and I must turn off the computer... :'(

any suggestion ?
thanks a lot !!

[psw]

When I got BSOD SESSION5_INITIALIZATION_FAILURE 0x71 (0 0 0 0) due to some NTFS problems I loaded successfully only uning 'Last Known Good' configuration.

[kirby_blue]

thanks...I tried your method but I also get the blue screen 

Yesterday, I could still boot in safe mode, and I tried to launch a chkdsk on reboot...
Since then, it won't even boot in safe mode or in command line...I now get a blinking white '-' on black screen lasting forever

Hopefully I have Ubuntu installed on dual boot (I knew it could be useful someday
anything you think I can do from there ??
or maybe from a recovery cd ?

EDIT :
tried ntfsfix from ubuntu and no success
tried chkdsk from recovery cd....finds errors and corrects them but in the end, nothing changes...

[pk]

Use Recovery Console from your Windows XP installation CD (or recovery cd).

Check these files and replace them from your CD if they are different:
  \Windows\System32\ntdll.dll
  \Windows\System32\ntoskrnl.exe
  \Windows\System32\smss.exe

[kirby_blue]

I actually don't have a real recovery cd (laptop computer) so I took the files from another computer

and it changes nothing...

what I don't get is that my startup screen (wich is a customized one) is still the same even when I replace those 3 files... 
shouldn't it be replaced by the original windows loading screen ?

(if you have skills in virus analyzing, I can provide you with the malicious program )

EDIT :
Well, I did an online scan of the damn file. Bitdefender tells me it's Dropped:Trojan.Agent.AHZT while NOD32 and VBA32 tell me it's Win32/Rootkit.Agent.NFI.
It seems that it's a pretty new virus and that's probably why major antivirus softwares like avast! can't detect it yet...
As I can't find any info about it anywhere, I guess you professionals could give me a piece of advice ?
I'm not giving up !