Could be a lot of things, post details of your components to start off with, and any info that may be relevant, how long youve had pc, how full your hard drive is, could be something simple like you need to clear the junk off it or reinstall windows, we need as much information as you can give us though to be able to help you
If ur PC hangs or freezes..or stops responding..& this is happening again & again..u should clean & optimize ur PC..for dis u can try Advanced System optimizer..right now I am also using it for regular maintenance & I am very happy with it...!Its new and improved tools help u in this..
System Cleaner..they all r really helpful..
At least you can try this till u get to know about some other product..!!