I'm not sure I would bin an entire operating system for an intermittent problem, although many would make an exception for Vista.
A whole load of windows freezing problems are caused by drivers. The first thing to do is interrogate the logs the system keeps on itself.
Click on Start (the blue button on the bottom left of the screen), then control panel-system and security-administrative tools, then computer management, then event viewer.
In the event viewer window, expand the 'error' tab and see if there is anything consistently appearing in there, especially if the time stamps coincide with the system crashes.
You can't break the pc doing this & there are also many other ways to work out what's going on, but this is a quick & easy way to start. If ytou do see the same error coming up, then either google it or come back here for help.
Edit - it's a good idea to clean out the system with cc cleaner as mentioned above, uninstall any software not being used, or that has been installed recently - it may in itself be the problem. Also, if any hardware has been recently connected, that should be looked at too in terms of the driver software.