Above are all good ideas.
Give some more detail though. How olds your computer, and when is the last time you did a defrag/format. Chances are it's just way too much viruses/spyware. That can easily be fixed. A simple reformat will take all of it away. If you need to save anything just go into safe made and save it to a cd/flashdrive or put it one a seperate harddrive if you have one.
But it could also be your harddrive failing if you got something unreliable like Maxtor it could be going bad and it will eventually die, I'm not saying that's your problem but it's always a possibility.
1.) Go into safemode and run some virus/malware scans. If you can't even get into safemode. Put in your OS disc and format the harddrive. If you can't even boot to that then i would say your hard drive is probably done. You should be able to get a new one and fix everything. But before you do that you should take it into a PC Repair Shop just make sure it's not any thing else.
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