Hello if I may jump in here and offer some assistance.
First, here is a support link from HP for HP computers that have blank video screen issues at start up.
Go down this list to see if anything suggested on that page solves your problem.
I am assuming you had no graphics problems before your computer became infected, if this holds true, there are cases where Viruses and Trojans can corrupt the firmware that is written into various pieces of hardware, a computers chip-set or BIOS chip without the computer owners knowledge.
As I stated above, if your graphics issues started after your computer became infected, your computer MAY be suffering from this possibility.
Please follow some of the suggestions at the HP web page I linked you to above.
If nothing on that page offers a solution, or if you have any questions, please feel free to ask.
If I do not reply to your request within 24 hours, please feel free to PM me.