I acquired Virus Barrier as part of one of the MacUpdate bundles, and I've had problems with the real-time scanner, where occasionally, the virusbarriers start hogging the CPU to the extent that my Powerbook G4 was almost unusable. I've disabled real-time scanning, and let it run scheduled scans instead (it's never found a virus so far, which doesn't surprise me).

But I must say that NetUpdate is truly an annoying application. It must have been written by a Windows programmer, because instead of bouncing an icon on the dock when it opens like all well-behaved Mac apps do, it barges to the foreground to announce that updates are available. I click the OK button, click the Update button (why not make Update an option of the announcement screen?), enter my login password at the prompt, and it starts updating. I resume whatever I was doing (such as web surfing), and when the update is complete, NetUpdate bulls it's way to the foreground again to let me know that it's finished. I click OK to acknowledge that it's done, and it checks for updates again. On the occasions that it finds another update, I have to go through the entire process again, including entering my password, cursing it as it interrupts whatever I was doing. Surely this intrusive, cumbersome process could be streamlined.