Interesting...
The parameter
/noguiboot is most likely causing the problem. If you remove it (you may have to reset the attributes of the file first, because it's probably set to read-only / system / hidden and you couldn't store the changes otherwise), it should be fixed, most likely without any side effects (unless you have tools like SoftIce installed

).
However, avast! should be looking for that parameter and remove it temporarily... I'll check if it works.
What version of avast! do you have - 4 or 5?