More info is always helpful in helping you to solve these sorts of issue's ie. desktop or laptop?, type of operating system and also what you see on your monitor when this situation occurs!
Also, can you think back to anything you did in particular before the situation arose, ie did you install any new software or hardware etc.
There could be several issues causing you to suffer 'memory dump', including bad drivers, software installs etc. Do you get the 'memory dump blue screen'? A 'stop error' code?
You may experience the "memory dump blue screen" if you don't have a good video card (common with older laptops). This occurs if you have hardware acceleration turned on. Don't worry, there is an easy solution, turn hardware acceleration off so your computer is just using software acceleration.
To do this choose Start > Settings > Control Panel > Display, then choose the Settings tab and click on Advanced. Then choose the Troubleshooting tab and move the scroll bar to none.
Hope this helps!