I suggest you uninstall avast and install antivir.
You can download a sasser worm scanner from http://www.symantec.com/security_response/writeup.jsp?docid=2004-050114-1706-99
I really do hope you fix it, it gave me a hard time when I had no idea what it was or even what to do. You can start your pc on save mode by pressing F"number", it is not always the same. It might be F12, F10, F8 or F2, or look at your screen when you start your computer.
Keep pressing one of them and you should be seeing options after starting your pc, chose safe mode with networking...press yes. Then download antivir and a saaser worm scanner. And use this too : http://vil.nai.com/vil/stinger/
b4 uninstalling or installing anything.
download it and scan ur pc.