OK place the Avastclear file on the USB
Reboot the computer and repeatedly press F8
From the list select command prompt
At the command prompt type

wmic logicaldisk get caption,description,filesystem

This will then list all your drives including the USB

Caption Description FileSystem

C: Local Fixed Disk NTFS
D: CD-ROM Disc
E: CD-ROM Disc
F: Removable Disk
G: Removable Disk
H: Removable Disk
I: Removable Disk
J: Removable Disk FAT

Then type at the command prompt J: (as an example)
press enter
Type dir and ensure that avastclear is there
Then at the command prompt type
avastclear.exe