After reading countless posts...this is definitly a problem with vista's compatiblitly with other software/drivers installed on the computer...so i have found various ways of fixing the issue depending on what specific compatibility issue it is. I think i have fixed mine, al least for now. My windows was telling me that it was a problem with trendmicro software. i had all of the updates for pc-cillin, so i couldnt figure out what was the problem. finally i found this link:
on the support page and downloaded the hotfix that they recommended.
my computer shuts down properly now, but I am sure it is just a matter of time before vista finds something else that keeps is from shuting down.
hope this helps.