need help virus on xp media center 2005

well cant tell what virus it is it rejects windows defender so i cant update and its really severe
nothing works at the moment i was a member on here just couldnt remember what my pass is but i need help
i tried safe mode and it fails

Please download the current version of HijackThis from HERE

• Double click and run the installer.
  
• It will install to C:\Program Files\Trend Micro\HijackThis\hijackthis.exe
  
• After installing, you should get the user agreement, press accept and Hijack This will run.
  
• Select Do a system scan and save a log file. This will open a notepad file of everything Hijack This found, copy and paste it back here.