u must check all connections inside the case if u have the experience to do so, it could be a lose connection

OR it can be other and more serious matters like an overheated cpu or ram or even heat damage on MB/bios

i doubt a video card damage can result in such a thing,

but all in all since u say it DOES go to your windows login page i suspect a failed /failing fan on cpu... most systems have an automatic setup that tells them to restart/shut off pc @ a certain temperature on cpu and in such case u should take it for repairs or if u r familiar and experienced in handling components inside of your pc, u may analyze your cpu and its fan. same thing can happen with video card chip and/or bios on certain mobos


for a very slight chance it could be a virus as well, with information u provided and with me sitting far away from your pc i cant really establish a cause but tell u what i know could have caused this to best of my knowledge

* none of this info is guaranteed, act @ your own risk- this only comes from me being a savvy pc user since age 8 and f-ing around with different pcs all this time *