Vlk, actually i ran into similar (if not the same one) myself too.
Few days ago i repeatedly scanned several samples and all the sudden i got message like this for context extension scan. Ok wtf, then i want to access tray menu and avast! icon just dissapears. I want to launch avast! with main exe, same error. Tried log in, log off, nothing same error over and over again. Tried manually restarting services, nothing it just wouldn't do it.
Started ashDisp.exe, it was running but no tray icons.
Then after i rebooted my PC everything returnd back to normal.

WinXP SP2 system here... Don't really know what caused this huh.