I had this problem before on my Compaq Deskpro, it would get as far as "windows is shutting down", and that was it, but i didn't even no what avast! was back then, i doubt it is because of avast! though, well not directly anyway, well to cut a long story short, im pritty sure i had to reformat my pc, but i can't be sure.
Basicly the weakest part of Microsoft Operating Systems is the fact that dlls are so easily replaced. They should be protected and Microsoft should not allow anyone to write the dlls. Because Microsoft is using Public Domain DLLs they probably cannot restrict them. Adding new devices or software often bring up the dialogue box "An older version of this file is being installed...." at this point you should always chose save the original (newer) file.
--lee