EDIT: Lets first run diagnostic on it to make sure that you have no hardware problems. When you press the power button start tapping the ESC key and the BIOS menu will open. Select diagnostics and test everything for errors. If the hard drive fails or it says you do not have a hard drive, then you're going to have to get a new one and order the recovery DVD from HP. If everything passes, You either have a virus or a corrupted OS.
Working on the theory that you have all your files and pics stored in your Library folders (please tell me you do) it will be easy to back everything up. I prefer to make a straight copy of the folders to a thumb drive or external drive. Plug in the backup medium and then open My Computer. On the left hand side you will see a panel with a scroll bar. Scroll up until you see Libraries. Under that you should see your 4 folders. Click the little symbol that looks like an arrow beside Documents and it should open showing My Documents. Right click it and then click copy. Reopen My Computer and open your back up medium. Right click once on its open Window and then click paste. This should copy the entire folder to your backup medium. This could take some time depending on your medium your using the amount of files you have. Repeat these steps for My Pictures, My Music, and my Videos.
There other methods you can use. Windows has a backup program but I have never trusted it. It has failed me many times. If you have another method for backing up that you feel comfortable with you should use it. The reason I prefer this method when doing a factory reset is because everything is stored in its regular format and easy to see that it is on the backup. Other methods tend to compress files and make it hard to see what is there and could cause problem when you go to retrieve your files.
Once you have recovered what you need you can reload your OS. Unplug everything from the lappy except the power.
If your resetting a lappy, make sure you have the AC adapter plugged up. You do not want this process interrupted. This should take one or two hours. It is best to check on it to make sure everything is going fine.
If you can boot into Windows, click on start and then open your control panel. Then click Action Center click Restore your computer to an earlier date. Then click on Advance recovery and select return computer to factory conditions. Answer any question and the system should reboot. Answer a few more questions and then it should go on autopilot and reset your system back to factory conditions.
If you have Windows 8 or higher you have to be really fast pressing the F8 or any of the other FN keys after pressing the power button. The boot sequence is so fast it will ignore the FN keys and go right into booting Windows. The best way is to have a finger above the FN key and then depress the power button. As you release the power button start tapping the FN you need.
If you can not boot into Windows then try Advanced boot options. When you press the power button, start tapping the F8 key. You will see the advance boot menu. Select repair your computer. (This option is available only if the tools are installed on your computer's hard disk.) Select your language and then enter your user name and password. Then click on (brand name) factory Image Restore and next. Answer the question and it should start the recovery back to factory settings.
If you can not access the advance boot options then try recovery from the recovery partition.
When you press the power button start tapping the ESC key and the BIOS menu will open, select the recovery mode. This will reset your puter back to factory settings.
This is what a BIOS menu will looks like. Yours might be different, I am posting this one to give you an idea of what to look for.
F1 = System Info
F2 = System Diagnostic
F9 = Boot Device options
F10 = BIOS Setup
F11 = System recovery
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First off lets make sure that you only have one AV running. The suggestion I made only required one AV to be installed and that was Malwarebytes. It will run along side other AV without conflict. RKill should have been run from a thumb drive. ESET should have been run in the safe mode from there web site. The other AV I suggested should have been booted from a thumb drive or CD. If you only have one AV and Malwarebytes running and can not get rid of your problem after following my instructions then your best bet is to back up all your files and do a factory reset. Sometimes that is your only choice.
If you post your make and model I can walk you thru backing up and resetting it.