PLEASE LET ESSEXBOY GUIDE YOU......I'm just giving some ideas......he is expert.....he needs to be the one to step you thru things.
In other words, don't get anxious and do something that could cause worse things and no way to undo.
OK. I won't do anything until essexboy gives me the go ahead. I'll just leave windows xp alone until then. I installed the windows xp 64 bit operating system into my VMware workstation 10 virtual environment using an iso disk image file from microsoft website. It came with instructions to create a bootable CD from the image file, and I can also transfer the image file and use it to boot from usb. I can't boot from a CD because the computer hosting all 3 of my operating systems doesn't have an optical disc drive. I don't know if I can boot from usb and run essexboy's program from usb at the same time, so I'll just wait for instructions.
Also, the avast install is not curupted. The same day I ran into this problem, I used the same installer download to set avast up in windows 8, 8.1, and 7. All were a success except for windows xp 64 bit.
I'd go ahead and create bootable USB Stick with the image.....also copy FRST (64 bit version) to the stick too.....root directory.
Make sure to get 64-bit version of Farbar:
http://www.bleepingcomputer.com/download/farbar-recovery-scan-tool/Go ahead and boot to the USB stick, get into recovery console, command prompt and run FRST64.exe.....post all logs generated.
Besides FRST.txt there should/may be one called Addition.txt.....both will be in directory FRST EXE is run.
In order to find your USB stick drive letter.......at the command prompt type the following:
notepad and press Enter.
The notepad opens. Under File menu select Open.
Select "Computer" and find your flash drive letter and close the notepad.
In the command window type
e:\frst64.exe and press Enter
Note: Replace letter
e with the drive letter of your flash drive.
The tool will start to run.
Lastly, I said that the Avast install was corrupted meaning that "during" the install on XP 64-bit something went wrong....not that the file/installer was corrupted. Also, if this gets cleaned up there is new 9.0.2016....make sure you go get this latest version/installer.
http://forum.avast.com/index.php?topic=147778.0OK....now lastly.....while in command prompt I'd save any data files you want off to the USB stick just in case.
If all the above works hopefully Essexboy can help.....but fallback may be to do repair install of XP.