Autosave files are stored in the same folder as the original drawing or in the same directory as the QCAD3.ini file if the drawing has not been saved yet. This later location depends on the platform. Under Windows, it's something like:
C:\Documents and Settings\<Your Login Name>\Local Settings\Application Data\QCAD (XP) or C:\Users\<Your Login Name>\AppData\Local\QCAD (7,
Note that QCAD automatically handles auto save files. It asks you if you want to open the autosave file if you access the original drawing and a newer auto save version is available.