Boot from a DOS/Windows floppy, make sure your CD-ROM drive still works (you will need DOS CD-ROM drivers and mscdex.exe loaded from config.sys and autoexec.bat - ask if you don't know how to do this).
Then fdisk, reformat, and reinstall everything.
That's the only way to make it "yours" and be legal with the software.
Otherwise, you have lots of registry editing to do, and even then you probably can't get EVERY trace of prior owners.