Ok, don’t panic. :wink:
I do believed that ComboFix has been detected rootkit but avast wanted as well to peek in that what CF looks/checks and thus is results of confusion.
ComboFix is extremly powerfull on XP systems. Again, on the other hand sometimes it is hard and run as well …

1. Delete the current copy of ComboFix (drag&drop icon into Recycle) and download new, fresh copy of the tool.

2. Disable avast using these guide:

[*]Right-click on the avast! icon in the lower right corner of the screen and choose Open Avast! User Interface.
[*]In the window that opens on the top right corner, click Settings.
[*]In a new window that opens, choose the option Troubleshooting, Uncheck Enable avast! self-defense, and click OK.

[*]Right-click on the avast! icon in the lower right corner of the screen and select avast! shield controls .
[*]In the menu that appears, choose Disable Permanently. When you are prompted to turn off security, click Yes.

Note: Do not forget to turn on this option after the cleaning.

3. Try to run ComboFix again and follow the prompts.

If you fail again to complete scan with ComboFix, then restart the system once again, follow the step#1 (download fresh CF), disable avast
and try to run ComboFix from Safe mode (boot Windows XP in safe mode and run CF from there).