No single product is 100% foolproof and can detect and remove all threats at any given time. The security community is in a constant state of change as new infections appear. Each vendor has its own definition of what constitutes malware and scanning your computer using different criteria will yield different results. The fact that each program has its own definition files means that some malware may be picked up by one that could be missed by another. Thus, a
multi-layered defense using several anti-spyware products (including an effective firewall) to supplement your anti-virus combined with common sense and safe surfing habits provides the most complete protection.
There will be no conflicts using WD with Norton.
Of course, you can always uses other alternatives.
Bleeping Computer's Freeware Replacements For Common Commercial AppsBleeping Computer's List of Virus & Malware Resources