It's not so complicated if you have your windows installation disk (often bundled as a 'restore disk' with factory-built computers). Just boot up the computer with that sucker in the CD-ROM (hopefully the CMOS is set to check the CD-ROM as a boot device before the hard drive) and follow the instructions to perform a complete windows installation. The option to reformat the hard drive prior to installation should make it self evident.
If you don't have a windows installation disk (or 'restore disk') then go buy one. Shell out $100 for Windows 7 or something. Frankly, it's a stupid thing to not have a means to reinstall your operating system, because every once in awhile a computer manages to get FUBAR enough to need its OS reinstalled, and without such a disk you're SOL.
You're probably being a bit paranoid about the stuff the previous owner had on there. PC software piracy is out of control specifically because enforcement is so unable to track it down in a timely manner. You really think they're not only going to track it down, but track it through another purchaser to you? A whole other order of unlikeliness. Even if it did come down to that, you've a pretty good excuse.
It wouldn't be all that hard to make the computer yours without all the work involved in reinstalling windows. First, just go over to Control Panel and uninstall any software you don't want. Next, go to Add/Remove accounts and add account, make sure it has administrator access. Log on as that account, it's yours. Now delete the old account. That should be a pretty clean way to get rid of his stuff.