no audio decompressor error in virtualdubmod

I have ripped 5 dvd files and want to convert them to avi, i know i can do it because i have done it before. This morning when i opened the files in vdubmod and then tried to save them i got the error message NO AUDIO DECOMPESSOR COULD BE FOUND TO DECOMPRESS AUDIO FORMAT. I'm wondering whether i have lost a codec somewhere. Can anybody help?

Try installing and see if it works.

Somehow my AC3 filter was wiped from my pc, i just re-installed it and everything seems to be fine. I dont know why it disappeared like that, i think i must be running an unstable system. Oh well i guess i've got some tidying up to do.