Which splash screen are we talking about ?
The Simple User Interface (SUI) isn't a splash screen, it requires you to close it at the end of the scan or are you saying the SUI disappears ?
A splash screen is something like what appears as a program loads and disappears once loaded.
If it is the SUI disappearing, does it minimise to the Task Bar or completely disappear. Are there any errors in the avast log viewer, Warning or Error sections ?
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.