I am running Mac OS Leopard on my Macbook Pro. Using Bootcamp I run Windows Vista 32bit too (I am ashamed of that). Lately I have been seeing some weird behavior with my Windows side. the screen just freakin freezes, the only option is to hold the power button down and restart, Ouch!! After reading few blogs I kind of realized that this might be an issue with the NVidia graphics card and Bootcamp drivers.
So I got the latest Mac OSX Snow Leopard CD. The installation could not be smoother than that. Absolutely stunning. Booted the Windows side, started installing the new Bootcamp 3.1. It upgraded several new drivers for NVidia, ATI, Intel …. And almost at the end, booom! Service “KeyAgent” could not be started. Of course nothing is straight with Windows I said. So I used the most powerful command know to a Windows user, RESTART. I restarted twice to get the same error.
Then I realized Vista being a really clumsy OS, there is something annoying in it called the User Account Control. I keep it off all the times. So I turned it on, restared again, and this time guess what, the installation worked. It seems that the installer is trying to modify some bits which the UAC was not allowing. Not sure though.
Try this if you are stuck with this error. Honestly a better option would be to get Windows 7 or atleast upgrade to XP.