Computer restarts itself now wont..virus??

Hello,
Hi I have the same problem at the moment when i start up my computer it stops at a black screen with white writing that gives me the opportunity to start up in safe mode, normal mode, last known working mode & another one with networking...I have tried all of these & everytime the computer switches itself off then straight back on again & i am faced with the same problem all over again. The computer has turned itself off & restarted itself in the past but i've always been able to get past this stage & have had it up & running. The only difference is that this time before it started not letting me on there seemed to be a virus on it as whenever you went onto the internet & tried to get onto ebay/facebook or any website a red screen came up & was warning me they were all unsafe basically it was trying to get me to buy some sort of virus software stuff from america (was in $) basically my partner has obviously clicked on something he shouldnt & given it a virus that wont let me look at anything but the search page on the internet.....Is my computer broken if so is it fixable what should i do? sorry if i sound a bit thick but I'm not great on the workings of computers x