Recently I noticed that my PC was thrashing around with a lot of disk I/O while I was trying to work. When this happens, I usually find that I inadvertently left Automatic Updates on or a virus scan is running, but this time I find that a process in Task Manager called GoogleToolbarNotifier is using up quite a bit of CPU time every second or so. Self-Googling Google, I found this site that explained that this is basically Google's answer to Microsoft's attempts to set the default search engine to MSN. Since I didn't want my PC to be part of this virtual battleground, I used the listed instructions to delete GoogleToolbarNotifier, and once again my PC is tranquil and speedy.
