Hello and welcome to BleepingComputer.
The reason that certain antivirus tools need to be uninstalled, is that they cannot be effectively (completely) disabled and they detect and remove certain parts of Combofix, which makes the tool unreliable in best case. In worst case, combofix might do its job only half, in that way causing serious problems.
To avoid any of such problems, combofix will not continue if those applications (CA/AVG) is installed.
I hope this answers your question.
Edited by elise025, 29 November 2010 - 02:26 PM.
"Now faith is the substance of things hoped for, the evidence of things not seen."
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Malware analyst @ Emsisoft