PLEASE HELP! Google search links redirect me to Ads and other websites!

So I just got this computer three months ago, about the second month it began redirecting me from google search links to other ad sites, search sites, and just random sites I did not want, I could not find anything to fix it, I stopped using it, then I took it to get repaired from where I bought they said they did not find anything, I am completely helpless I bought this computer for college which I'm about to start in August 24, but what is it good for if it can't go trough a simple google search, I do not have the money to buy a new one and my warranty has expired, it started out fine, I don't why this is happening.
It is also starting slowly, sometimes it won't even start, and everything takes a while to open.
I use Firefox as my browser, the latest version, I have Mcafee security center, It's a Dell X, I just don't know what to do, I hope I can get help here.
I followed the eight steps, did everything as well as I could cleaned my system twice etc. I am not computer savvy at all so bear with me.
Attachments are attached, please tell me if you need any more information.

**Reply removed due to a forum issue.

Sorry taniagisell I just had to remove this reply due to an outside issue. Someone will be along later to help you

Thank you for the help, the Combofix log was completely empty, I don't know if that's normal?
when I try to open it, it says it is being used by another application?
But the Hijackthis report did work.

just checked Google search links and they are working! I've checked and checked and everything seems to be working fine!
Thank you VERY MUCH, I appreciate your help so much, I will definitely come back here if I ever have any problems again.
I just wanted to know, how this happened exactly? I don't think I've gotten into any websites I really shouldn't and how can I prevent it from happening again?
Should I leave Combo Fix on my computer? am I good to go to install new programs?