I recommend this Virus Bulletin article, specially to virus analysts.
I would like to see some comments from Karel 
Is avast prepared to this? I mean, only signatures seem not to solve the problem…
Malware writers have always striven to create malware in such a way that it will evade detection by anti-virus software. Code obfuscation is one of the methods they use to achieve this. Over time, obfuscation techniques have evolved from simple encryption to polymorphism, metamorphism and packing. The latter is becoming increasingly popular these days, with lots of packing tools available to malware writers, and more of them being created each month.Dynamic translation can be used to achieve generic unpacking with good speed performance. However, detecting generically unpacked malware using an existing signature set is not guaranteed to succeed. New signatures can be extracted to mitigate this problem. Combining DT with specific routines to rebuild the original binary from an unpacked image is yet another solution to be explored.
http://www.virusbtn.com/virusbulletin/archive/2006/03/vb200603-packed