When is reformated my com 2 months ago, i did not install any anti-virus or related software. So after a while, this spyware(or virus or trojan) change my home page to sum stupid search engine

no matter how much i tried to use different anti-spyware likeAd-adware or spy bot, it was to no avail. Then a week later, when i log on to hotmail, it change to that same website again. Another week later, others sites that i needed to be log in also turn to that website.

Another problem with my computer( don't know if they both problem are related) was that one day my wave volume started to keep on going mute, i tried restarting and is still the same. The next day, it was my master volume that started to go mute.
the worst came the next day, where i was not able to open some program, as my com will crash.

I end up reformating the com. Problem solve.

So can any1 tell me what the problem is and how can i prevent and solve them