How can I fix a computer with many viruses?

I want to try to fix my old Gateway MA2A computer running Windows XP myself, and I'm a complete noob at this. Should I replace the hard drive? If so, will this one work?



http://www.tigerdirect.com/applications/



If I replace it, will that change the operating system?

Any help would be much appreciated.How can I fix a computer with many viruses?
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