OK, I think we found the cause... and made a simple workaround, but it's not very nice.
The reason is the following: the operation of "opening an archive" is rather slow for some of the archives (such as CAB), and cannot be interrupted. Actually, opening the archive means extracting the whole archive to the TEMP folder (if it's done in a different way, scanning the CAB archives would be terribly slow, just as it was in avast! 4.0). Now - when the screensaver starts opening the archive and right at that moment, the screensaver is interrupted (by moving the mouse or typing something), the process is given a while (10 seconds, to be precise) to terminate and if it doesn't, it's "killed". By killing the process, the temporary files are left there.
For the future version, I changed the behavior a little - such that the scan is never killed this way; if you interrupt the scan when the archive is being opened, the screensaver just disappears, but keeps working on background until it's done with the operation. So, you interrupt the screensaver by moving your mouse, the screensaver disappears, but avast! still keeps on opening the archive for a while (so, you may see a heavy disk activity for a minute or two). I know it's not a perfect solution (which would be making the opening operation interruptable - but that would require rather heavy changes to the code).
An additional solution I'd suggest - exclude these big CAB files from the scanning
