Back Up your Important info
Format your Hard Disk, Install Windows XP Professional with SP 2, and all the latest Security Patches from Microsoft
Install a good Anti Virus Software ; Be Sure to Update it and to RUN a Full Scan just following installation and keep it updated regularly.
Install Spyware/Adware removal tools/software , update to the latest definition files and keep it updated regularly, run systemically periodic scans
Your hardware is good enough for Windows XP, Use it - While installing be sure use NTFS.
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On a long enough timeline, the survival rate for everyone drops to zero