My computer won't let me put antivirus software on it!

Please help me. My computer/laptop won't let me put antivirus software on it! It has problems and sometimes will just automatically shutdown itself and crash or lock up. I need antivrus software on it badly! I put the same software on my desktop and it worked fine, but when i try and put it on my laptop it always says some kind of error for any kind of software. I know this might sound like I should go take it to a "Geek" but i don't have that kind of money right now! Please help me. thanks.