This is one of those problems that could have any one of a number of causes, the most common one being driver problems. Disconnect any external devices, such as routers, printers etc. and try again.
You might try performing a clean boot; open the start menu and type type msconfig in the general tab, choose "selective startup" and uncheck "Load Startup Items". In the "Services" tab, choose "Hide all Microsoft Services", then click the "Disable All" button. The restart the computer.
This might give you an idea as to what's at the bottom of the problem.