When I try to update the virus database I get an error message that reads, “Setu000000a0 has caused an error in SETU000000A0.VPU. Setu000000a0 will now close.” What’s wrong? 
I posted the same problem (see “Performed Illegal Operation” above) before reading your message, and then solved it myself.
I downloaded the setup progamme again and installed it over the top of my existing version. This only took seconds and once installed iAVS worked fine again.
Hope the same works for you
Check the length of the affected file, i.e. c:\program files\alwil software\avast4\setup\setu000000a0.vpu
It should be around 295kb. Is it?
Thank you crowterman and kubecj for your help. I downloaded the setup file again and everything works fine now. ;D