My HP computer is frozen on the start up screen, what do I do?

My sister downloaded a lot of material yesterday to get credits for some type of online game and a lot of things started popping up on the computer. I then started mass uninstalling the programs. When I realized a program called geneio wasn't on the list of installed programs I tried to uninstall it manually by deleting the folders. It said it was still open therefore I couldn't remove the files. I shutdown the program's processes via taskmanager but still no luck when trying to delete it. I also accidentally cancelled an unknown process as well. It said HKCU or something like that. Ending this process didn't do anything to the computer I think. I then opened up msconfig and unchecked all the programs that opened up at startup. I restarted my computer, and right before it shutdown avg said there was a Trojan on my computer. The startup screen that shows what some of the F-keys do (For system restore and stuff) popped up. Then a black screen telling me that a USB was overrun popped up and told me the computer will shutdown in a couple of seconds. I unplugged some things and started it up again but still the same message. I unplugged everything at that point and left the computer alone for about a day. When I plugged everything back in and started up, the screen showing the f-keys for system restore and stuff popped up. I tried doing system diagnosis but no luck. Then after trying to do all of them, nothing happened. The screen has been frozen in that state for hours now. What do I do?