How do you diagnose constant crashing issues?

My recent installation 10.10 on my laptop keeps freezing/crashing on start-up after working perfectly for one day.

The system just freezes, like a screnshot, and a restart is required. It happens directly after login or just as any application is selected.

Nothing new has been installed. I have just moved from XP so I would be very grateful if some one could please explain to me, as simply as possible, how to do the following...

1. How to diagnose the problem so it can be fixed, remember it may have to be before the log in.

all help is greatly appreciated.

When i have a problem like that i normally do the following:

1. In the grub menu select the RECOVERY MODE
2. When the blue menu appears select to run as root in the terminal/shell
3. When you login in the shell as root type dmesg to see any problem of devices when you were loading the system.
4. To see a more specific summary type cat /var/log/syslog which will show you EVERYTHING when you were loading the system.
5. You can add | less to the command above like this cat /var/log/syslog | less so it shows you the info and you press down or up to see the output and press Q to quit.
6. If everything shows good type startx which will start the gui desktop for gnome where the actual problem appears to be.
7. startx will mention what problem he is having in the terminal so you can fix it.
8. If by some chance the computer gets stuck when loading startx press CTRL+ALT+F2 or CTRL+ALT+F3 (OR F4,F5,F6) and type in your login user and password. Then type ps -ex so you can see the ID for gdm process or startx and just killall -9 startx / killall -9 gdm or kill -9 ID or whatever process it was loaded when you started startx. This way you will the process and the system is not stuck anymore.

With that you have dmesg, cat /var/log/syslog and startx to see where the problem is. At least for a quick look.

Let me know if it helps somehow. This is just a quick check for problems.