I too am slightly confused.

i tried to enter safemode (using F8) still i cant enter
Do you mean that if you boot into safe mode it will not do a restart into normal mode

Or will it just cut off in safe mode as well.

It sounds very much like malware (blaster? sasser?)

If running "repair" from your installation disk and/or going back to a recovery point BEFORE you had the problem does not work, please let me know.

There is a way of stopping it shutting down so that you can get rid of the malware. At the command prompt line enter shutdown -a

If you get a message like: "Unable to abort the system shutdown because no shutdown was in progress" then it may well not be malware.

Remember, there are a number of hardware problems that show the same symptoms. (Overheating, RAM failure, power supply etc.)

Please post your answers and we will try to help.