I have an Athlon64 3500+ (socket 939), running WindowsXP SP2 RC2, and I'm running Avast Pro!
My boot loader does have the /noexecute flag, and DEP is enabled.
I just tried running a boot-time scan, and every file returns a 41200 error.
I then loaded Antivirus, selected c:\windows, and tried to scan. Every file returns the internal error you mentioned.
Are there any other tests/configurations you would like me to run?