i did a clean and full non-nlited XP sp3 install and installed the drivers via device manager one by one (not going through setup.exe).
at first i have the same problem i was freaking out fearing that it may be a hardware design issue because the problem seems to be happening on linux as well (from googling and reading other forums), fearing that the machine may be faulty
but then i tried doing a proper full install of the realteck driver package (the one you download from the acer-euro ftp site running their setup.exe) and mic works again on msn and skype, so you may want to try that