Have a look in the Event Viewer for any errors during boot.
To open the Event Viewer go to Start > Control Panel > Administrative Tools > Event Viewer. Alternately, go to Start > Run and type in "eventvwr.msc" (without the quotes) and press Enter.
Check in all three categories (Application, Security, and System).
If you find an error that occurred at the time right-click on it and select properties. Copy the information in the window and post it back here.How To Use the Event Viewer