Yamaki, problem is not on BootScreens but, God, why did you install NAV and avast at the same time in your computer???

You can't uninstall avast! by command line (dos mode). You'll need to get access to the Registry, NTFS read/write for DOS program,, etc.
But, strange, safe mode give you a BSOD (IRQ_NOT_LESS_OR_EQUAL stop screen)...
Can you go to C:\WINNT\CONFIG.NT file and list the contents?
Can you edit this file (or at least, copy to a floppy, edit it and put them back there?
If so, try to see at the bottom, after the comments the 'active' lines such these ones:
dos=high, umb
device=%SystemRoot%\system32\himem.sys
files=40
device=x:\short_path_to_avast4\aswmonds.sys
If you add a REM before the line:
REM device=x:\short_path_to_avast4\aswmonds.sys
Boot, will it solve the incompatibility?