When you say "got" your system, did you buy it pre-installed with Win 7 or did you install Win 7 on your system?
Have you check the event viewer to see if you can see anything that might lead to the problem? Open Start menu and type eventvwr.msc in the search box. If you see errors or warnings, expand those and double click to get information about them.
Type msconfig in the Start Menu Search box and go to the startup tab to see what is starting up. Try unchecking some items so they won't load at boot to see if you can find the problem.
Does this happen if you boot into safe mode?
Are all of your drivers up to date and have you loaded any software that might be older?