Boot up and OS problems..?

hello there,

My computer is slow to boot up and sometimes doesn't boot at all. It will freeze before the OS even loads. If it does boot up, it will freeze on simple tasks like surfing the internet. I'm running windows XP. I've had this computer for almost two years and this problem came out of nowhere. I think it is a software error.

Thanks in advance for any help,
Techbrain55