Lilio, há pelo menos dois problemas aqui.
O avastUI.exe é a interface do avast! Você tinha outro antivírus antes do avast? Qual? Como o desinstalou?
Poderia dar-nos o nome do rootkit e o caminho completo do arquivo infectado?
Devem estar nos logs do avast!
Sugiro também um roteiro de limpeza (em inglês):
1. Clean your temporary files.
2. Schedule a boot time scanning with avast with archive scanning turned on. If avast does not detect it, you can try
DrWeb CureIT! instead.
3. Use
Comodo Cleaning Essentials (CCE), or
MBAM, or
SUPERantispyware to scan for spywares and trojans. If any infection is detected, it is better and safer to send the infected file(s) to quarantine (Chest), rather than simply deleting them.
4. Test your machine with
anti-rootkit applications. I suggest
avast! antirootkit or
Trend Micro RootkitBuster.
5. Read
these instructions and provide more info with the logs generated.
But, please, do NOT post there, open a NEW thread for your specific problem and help us to help you.
6. Clean your Hosts file (replacing it) with
HostsMan tool.
7. Disable System Restore and then reenable it again.
8. Immunize your system with
SpywareBlaster.
9. Check if you have insecure applications with
Secunia Software Inspector.
If the infection avoids booting the computer, take a look here
http://forum.avast.com/index.php?topic=79107.0Further instructions to get clean from Essexboy here.