system security

hi i have pops ups and ballons popping up telling me to download the antivirus. but i have been told its a scam i am not very technical of very good with comuter jargon i have tried to go to add and remove programs but it wont let me ive tried to download things to help me get rid of it combifix but it wont let me its keeps popping up then freezing things then crashing the internet please please help its driving me crazy!!!!
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Please download the current version of HijackThis from HERE

• Double click and run the installer.
  
• It will install to C:\Program Files\Trend Micro\HijackThis\hijackthis.exe
  
• After installing, you should get the user agreement, press accept and Hijack This will run.
  
• Select Do a system scan and save a log file. This will open a notepad file of everything Hijack This found, copy and paste it back here.