You can add such filtering preset (in dropdown list there are filtering presets, not avisynth functions):
1) press cogwheel button near filtering while video is not loaded;
2) press + (add profile) and name it as you want;
3) write "FlipVertical()" there and press ok;
Now you will have such filtering preset (also it will save if you open other video or close program), so "fixing" flip problem would be in two clicks.
P.s. I'll ask people, who are working on Xvid4PSP 5 to add option to add FlipVertical () automaticaly, when opening video via DirectShow...