If the problems started appearing just after installing broadband, then that has a good chance of being the cause.
Click Start, Run, type:
and press Enter.
Look at the errors of the System log. Double click an error and read the information. It may not be comprehensible, but see if it gives you any pointer. If the error appears to be the same most of the time, that's what you should concentrate on.
You can also right click My Computer, Properties, Advanced tab, Startup and Recovery, settings. Under system failure, untick the box for automatically reboot. click OK.
Next time the computer crashes, it will go into the blue screen of death, but you can write down the error message.