Can you start it in safe mode? ... to get there try this -
1) When you boot to the login screen, hold down the SHIFT key and click on Restart. This will take you to the troubleshooting option screen where you can enable Safe Mode.
2) Click on the Troubleshoot button.
3) Select Advanced Options.
4) Now choose the Windows Startup Settings option.
5) Finally hit the Restart button.
This should let you into the Advanced Boot Options screen. Choose Safe Mode (using the arrow keys) and hit the Enter Key. When it eventually opens dont worry if the icons look big and the screen shows wierd colours - it may be using different graphice settings in Safe Mode.
If you get there and you can see a desktop and the start button works then it is possibly a startup program/service that is causing this issue - perhaps you installed new software, or a driver has been updated. You can try now to use System Restore to take oyu to a time where you know the machine worked. I prefer to use the restore point before the date you know works. It might also be worth performing a virus check before the restore - assuming it lets you.