I had this issue with Windows 7 Ultimate 32 bit and Avast 8 Free.
You need to go to safe mode and issue a command to delete the aswtdi service. If you don't go to safe mode avast will prevent you to delete the service.
Start cmd as an administrator. Issue the following command:
sc delete aswtdi
Reboot and enjoy.
I have tested it with Windows 7 32 bit, I don't know if it will work with Windows 8.
Disclaimer: Use at your own risk. The risk is fully of the user.