Not with Windows, no. You'd need 3 computers, and use one as a domain server. Even then it'd be 'fake' floating profiles, where you only get a small subset of data shared between systems -- My Documents, Bookmarks, maybe Desktop. Basically everything in your C:\Documents and Settings\$USER\ directory. You would not have the same software, individual system settings, etc. As far as I know, true floating profiles are only available through UNIX-like systems such as Linux and when using thin clients (which are usually some form of UNIX anyway).
Along the line of thin clients, I guess you could use the more powerful of the two systems as the "master" system and run VNC on it. Then you could log into it remotely from the other machine and manipulate your master system's desktop. It would be slow, ugly, and limited (you wouldn't be able to do anything even remotely multimedia-like such as flash, videos, games, music, etc.) but would allow you to have basically only 1 system to work with and you could access it from any system/OS. I guess it all really just depends on what you want that functionality for in the first place.