Was this what you tried ?
If you have Windows XP/Vista:
C:\WINDOWS\SYSTEM32\REGSVR32.EXE ACTSKIN4.OCX
If you have Windows XP/Vista x64:
"C:\WINDOWS\SysWOW64\REGSVR32.EXE" ACTSKIN4.OCX
Press OK (or Enter).
A message saying that the file was successfully registered should appear...
For some reason, ActiveSkin (the 3rd party library avast uses for skinning) does not seem to be working on your system. As a workaround, we may disable skinning in avast - that should solve the problem. Program Settings, Common, and uncheck any option with skins mentioned (see image, avast-no-skin).
If you are unable to access the above you can do this manually. To do that, open the file using notepad <avast4>\data\avast4.ini, look up the [UserInterface] section, and add the following entry: StartWithSkin=0 - Save the changes, the avast self-defence module will ask if it is OK answer Yes.
If it is what you tried and it didn't work what was the error ?
In Vista with UAC, you may have to jump through some more hoops, by opening a command window using the run as function and log in as the administrator before entering the regsvr32.exe command line.