If there is still a malware issue on the old system, that should probably be addressed in a new topic in the viruses and worms forum, starting by outlining the problem, e.g. the reason for its use.
That said if there is a possible rootkit/MBR issue you should follow the full analysis process:
This needs further analysis by a malware removal specialist:
Go to this topic
http://forum.avast.com/index.php?topic=53253.0 for information on Logs to assist in cleaning malware.
Use the information about getting and using the tools and start your own new viruses and worms topic and attach the logs there, not in the LOGS topic.
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Given that you can't boot on that system (don't know if you have tried a safe mode boot), then it won't be possible to run the tools and get the logs. But an outline of the original problem and what you did and the resultant problem, a malware removal specialist may have a way to get into the system using a bootable CD, etc. However it depends on the circumstances.