Your computer has a symptoms of -ware attack, usually caught it unintentionally. In your case, whenever you open a browser, you tend to get redirected to a site where they suggest that you buy the product in order for the warez/virus to be remove. The problem is much more profound now depending on how long you had this problem. Usually on the first to second day when the warez enters your computer, it continues to let other unknown warez/virus. By purchasing the product from their site you only fix one of the many problems that was intentional on their part. Because your problem may be more than just one type of warez/virus you may not be able to get rid of the whole problem entirely. Assuming that you are a beginner users, I recommend you back up your system and perform a complete restoration and buy third-party software such as zone alarm or comodo firewall to help in your aid. These products are by far out performs McAfee anyday. Bigger companies don't try hard to help its consumer. Once they go big, they just mass produce to make money. Norton was good back then, so much for a Name brand now these days.
Whether or not you should do a complete restore on your computer depends on the complexity of the virus. If you decide to go ahead with the restoration process, main stream anti-virus product especially those are that are big brand names tends to under perform. For example, you have heard of brand as "Norton, McAfee and etc." The one that you currently own now is McAfee. There are many smaller company that does a better job than the bigger company. The disadvantage of having Norton and its counterpart is that, the softwarez itself slow your computer down because it takes more of your systems memory just to run it. Package program where you get anti-virus, firewall, anti-spam and etc. are a no no in a experience users book. Stand alone products out perform package bundle usually any day. For example on my computer, I am running a zone alarm fire wall with iolo system scan. For me, softwarez that shines are usually the ones that are less known; on the bright side they take up the least amount of memory on my computer. Giving me more memory to run my other applications and movie watching time.
If you continue with a complete restoration, I recommend you to have the following items install:
Beginners:
1) Firefox 3.0
2) Zone Alarm (free edition from Zone lab)
3) Iolo system doctor
4) Firefox plug-in a) pop-up blocker
5) AVG Anti-Virus
Novice:
1) Opera Browser or Firefox
2) Zone Alarm or Comodo
3) Adware Remove
4) Pg2
5) Systems tweak: e-mail me for a basic system tweak that even you can perform. I will give you the instructions. I recommend it also for beginners if they use their computers for multi-tasking.
Regarding your questions, if you are unable to load a third-party softwarez (anti-virus program) and fix your minor virus problem I would say your problem is a bit out of your hand.