I seem to recall that the Centaur CPUs tended to run slow under certain conditions.
Bear in mind that the Centaur is built around the idea of a relatively unsophisticated in-order execution core with really good branch-prediction and look-aside buffers, and a relatively small cache (L1 is 16K, L2 is 128K, there is no L3).
It seems likely that the AV software is swamping the cache and probably the memory bus, while also making a dog's breakfast out of the branch-prediction logic, and all of these things are contributing to your performance woes.
I'm not sure if they are compatible with your mobo, but the Nano and Eden series processors are pin-compatible with the C7, so IN THEORY you could swap for a higher-performing
CPU. I know the Nano and Eden have both been shipped in Vista systems, so one would think they would handle the load a bit better.