Try another reformat of the entire disk. Not the quick version either. Then another reinstall. Make absolutely certain that you have removed every hardware device usb from the back of the CPU tower except for keyboard, monitor and mouse. Allow the installation to complete WITHOUT activation/registration.
If you have made certain that everything has been removed from the CPU tower except for monitor, keyboard and mouse, then there would be no possibility for Windows to access the internet with an attempted registration/activation. This can be done at any time during your first 30 days after the installation. I can personally vouch for the absolute certainty that an infection can occur within three seconds of accessing the internet with an unprotected, unpatched operating system. Blaster worm was one of these that was quick to attack such a system, back in the day. If your installation CD does not have the Service Pack 2 already included, then you need to activate the Windows Firewall before accessing the internet. Ask me if you need to, for instructions how to do that.
Of course, once you have completed the installation properly, you then are instructed to re-attach the dsl cable or modem device for dial up so that you can access the internet. Once you do this, you are to immediately visit Windows update to install all of the updates presented.
After this, reinstall your antivirus software only. At this point, you more than likely are going to need another defrag. Complete that, then boot to safe mode and run a complete system scan. Allow the software to quarantine whatever it complains of. Boot back to your normal Windows user mode and reinstall any other programs you intend to use updating each one, one at a time. When you finish, you should then reattach any other hardware device you intend to use and do the same with it's related software.
Post back when you finish and let us know how the system is behaving so we can get you a free image creation software tool from which you can create an image file to use for your recovery in the future. Good Luck. Thanks!