I really need help with a virus that poses as an antivirus?

I get a windows bubble saying my system performance is at 47% and something else is at thirty-something percent. On top of that, I also get annoying pop ups that say I have a trojan and that I should download BestSeller AntiVirus. Now I also get error messages that say the same. I have tried to use McAfee but I found it slow and ineffective. I know of something called SuperAntivirus that gets rid of it for a short while but the virus just comes back. I have tried to put the computer back to the way I bought it but every time I try it says i have a fatal error followed by the code C000021a and I don't know what else to do. I need a FREE way to fix my problem so Norton is out of the question and I have Windows XP. Also, I've been getting pop ups on system defenders or security center or some other pop up on a system secrurity risk. Please help and feel free to ask for further information that might help me solve my problem. PLEASE REPLY ASAP

I really need help with a virus that poses as an antivirus?stinger

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I really need help with a virus that poses as an antivirus?virus scanner