Sometimes the Windows installer can have software "hidden" from it. A third-party uninstaller program is not as easily fooled. As evil as spyware is, "legitimate" companies like Sony (especially Sony!) are often worse and have more pervasive garbage installed on computer. For this reason, software like CCleaner (originally called "CrapCleaner" - funny!) was created. You can get a copy of CCleaner at http://www.ccleaner.com/download
IF you're still wanting to re-format the system -- not a process I relish, but one I know sometimes has to be done -- then either XP versions should work as a home user.
XP Pro is mainly geared towards computing-rich environments. For example, I use RemoteDesktop to access my server and my office computer both. That feature is only in Pro. As a software-only user, there's really no disadvantage to Home. I use MCE (the media-rich version of Home) on my laptop, and it's one of my primary video editing machines because it's got great specs (Intel C2D, 2GB RAM, etc).
When it comes to purchasing, you need the retail version. The OEM version is meant to be used by OEMs only. Buying the OEM version may cause you problems. Many of them are meant to be used in certain countries only, for certain computers only, etc. So MS may or may not help you when it comes time to activate, or if there are other issues to be had. It's not illegal to sell the OEM versions, but the intention for those was MS->OEM only, made for the specific needs of that OEM. Customers are large pay more, yes, sadly, but support costs are what has been bundled into the price. The OEM takes the support calls for an OEM version.
Hope that helps.