When I choose Avast as screensaver and choose example (in the display properties), it runs fine, but when I then click cancel to go back to the desktop without changes, it keeps running and I have to end it in taskmanager. This was also in 4.1 and it still is in 4.5.498 
Windows XP pro, SP2
(problem excisted already with SP1)
Works ok here. (Both 4.1 and 4.5.498) WinXP Pro SP2
Have you checked taskmanager under the processes list to see if it really was stopped?
Yes, the AVASTS~1.SRC closes when I cancel the Display Properties.
I’ve tried with different settings on the screensaver too.
Audio on a Screensaver is a bit odd… 
Sometimes, this happens while the module is scanning big *.cab or *.zip files.
If you use avast! Pro you could exclude that specific extensions in the exclude list for this task. If you use Home version you could, at least, do not check for scanning files into the screen saver module…
Are left behind some temporary folder at C:\Windows\Temp with a lot of files inside?
No large files. It even almost has to start scanning. It happens if I click “example” in the display properties and immediatly move the mouse or press a key also. So it doesn’t almost has the time to start scanning. If I leave it for a few minutes and than stop it the same happens.
Actually, I have improved the “interruptability” of some archive operations, so the problems with big CAB files shouldn’t occur much now. The only situation when the process may keep running for some time after the screesaver has been interrupted is when a big file is being processed - either unpacked from an archive (i.e. when the archived file is big; when only the archive (e.g. the CAB file) is big, it should be OK), or simply a big file is scanned (when “Test whole files” is set in the Screensaver task).
Eddy, what are your exact settings of the Screen-Saver task? What real screensaver are you using? (or, does it happen with any screensaver)?
I did a lot of testing with it and it happened with all settings/combination of settings.
Edit: The problem seem to have vanished with version 4.5.561 and sofar no problems with the beta (578) either.
You may also try the new beta - CAB unpacker is significantly improved there.