To change the Time Format in Vista from 24 hour to 12 hour follow the below instructions:
1)Go to START > Settings> Control Panel> Regional and Language Options
2)Under Formats Tab> Click "Customize this format..."
3)A new Window will open > click on the "Time" tab
4)Under Time Format, Choose "tt hh:mm:ss", then click Apply.
If you need to change an 12 hour format to 24 hour format just do the above procedure, and choose "TT HH:MM:SS" and click apply. Thats it... you can now have the time format that you look for. Soon you can see more technology tips in my blog.