I HAVE A BAD VIRUS OR WORM?

My computer has been very slow, crashing repeatedly, and getting random errors and it all happened so fast that I concluded it was not a hard drive failure at that because that takes time. But, Ive tried scanning with all the free virus software such as AVG, Avast, Spyware blaster, you name it, Ive tried them all! But nothing has worked. So after a few months I just stopped worrying about it and accepted my very SLOW PC but now its gotten to REALLY frustrate me!! So I decided to think back to what I might have installed that might have cause this.... Then it hit me, I kept getting notification from so called, Microsoft, telling me I need to install Windows Genuine, and then like 2 days after I installed it my PC started to get really slow, really fast, and that was the last thing I downloaded. But this is a very intelligent virus, or worm because it seems to go unnoticed by practically every virus detection software and my restore mode no longer works, my stand by option doesn't work, and my PC only boots in ONE safe mode which is directory service restore mode. In all the other safe modes, like the regular one, I get a blue screen that says, "Failed to load sptd.sys files" at the bottom of the screen. How do I get rid of this virus?