From the FAQ pages of Avast’s website:

Q: I have installed avast! 4 Home and every time I try to open it, I get an "unknown error message", "Application cannot load skin. Function usiGetSkin Failed". How do I fix this?

A:
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.

Results from a search I did can be found HERE