Windows reboots or shutsdown as soon as you login

If its caused by a virus or a crashing program this might work.
This is a tricky one ... n u got to be quick
Once you are logged in open Start->Run ( or better you can use the shortcut keys windowskey+r)
type shutdown -a and hit enter
The shutdown or restart would stop.



Now open run again and type msconfig and hit enter.
Here under the startup tab click "disable all" and click ok .
Your PC will reboot, upon reboot check if the problem exists, if its gone its one of the programs in the start up, you can find the culprit by checking the paths.
If msconfig does not help try running a virus scan after the "shutdown -a"