Is Dell's wbt.dat archive harmless?

On my Dell computer, Avast identifies 5 files within the archive c:\Dell\wbt.dat as unscannable because they are password protected.

Even if these are benign, what prevents malware from replacing this archive with it’s own spoof, password protecting it and thereby evading inspection from AV scanners?

No password protected archive is checked by AV scanners. That wont affect your security level simply because the contents cant be executed until they are decompressed. If the file was replaced, and the dell software did attempt to execute its contents, the malware/virii would be detected by the normal resident shield.