Where were you looking for it, for me in the c:\windows\system32 folder for WinXP pro.
If it isn't there, 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. You need to be on-line to do this.
If the avast! repair does not solve it:
You will have to register ACTSKIN4.OCX
Follow the instructions:
Select START -> RUN...
Enter the following command:
If you have Windows NT or 2000:
C:\WINNT\SYSTEM32\REGSVR32.EXE ACTSKIN4.OCX
If you have Windows 95, 98 or ME:
C:\WINDOWS\SYSTEM\REGSVR32.EXE ACTSKIN4.OCX
If you have Windows XP:
C:\WINDOWS\SYSTEM32\REGSVR32.EXE ACTSKIN4.OCX
Press OK (or Enter).
A message saying that the file was successfully registered should appear...
If that doesn't work try, uninstall, reboot, install, reboot.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.