HP--beware of any OS reinstalls, or changing the hard drive, motherboard, or CPU. If you do, it will know, and it won't like it. (Has to do with OEM license of Windows, and HP's recovery system) Same for Compaq. There's work-arounds, but it's a lot easier to not have to deal with it in the first place. (One of many reasons I'm building my own PC next time, and will be buying a "full" version of Windows for its OS, not an update/upgrade package from a previous version)
From what I've read, Dells don't have that issue.