It seems to be there can be 2 reasons of behind this problem
1) Your operating system / windows is corrupted and you need to re-install / fix it by pressing f8 key in start and then choosing Last known good configuration .
2) The power supply of your system is mal-functioning and need to be repaired / replaced.
At first i suggest you to use F8 options and if un-sucessful then use recovery disk if you have . if you are un-successful then visit any local computer shop for help.
Best of luck