After the latest round of updates from Microsoft, Vista's Deferred Proceedure Calls would go though the roof on certain network activity. Actually it was rather too high before the latest updates (as of the date of writing) but they increased exponentially after the updates.
Before the updates, and for some time now, on loading a webpage in either Firefox or IE, especially one that had heavy image content, audio would stutter (in Winamp and other audio programs) and the mouse pointer would jitter. On my Quad core Core 2 Duo Quad Q6600, Core0 would be at around 85%. The others would spike at around 50 to 75%. The fault was due entirely to high DPCs. Running Azureus would result in similar high DPC. It was suggested that the shitty audio drivers (Sigmatel) were to blame. This turned out not to be the case.
After the updates, the situation worsened, with Core0 pegging at 100% and the others reaching up into the 80s. I could do nothing else while a graphic heavy webpage was loading, or a torrent was downloading (seeding was without incident) as my mouse pointer lurched all over the place and the system was almost entirely unresponsive. Process Monitor revealed that the vsmon.exe (Zone Alarm) was doing an awful lot of something.
I checked for an update for Zone Alarm (7.5) and updated to version 8 and all these problems went away. Whatever Microsoft had been tinkering with had severely fuxx0red with what Vsmon.exe was trying to do, and it couldn't keep on top of things anymore.
Once again, Zone Alarm, that free, ubiquitous (and actually not too shabby) firewall had proved to be the culprit. Nothing to do with Azureus, Firefox, Sigmatel audio drivers, my chipset or Vista.