You can change it during install IF you make your own ISO (Sort like an Unattended install) and in theorie perhaps after installation (but may break the OS or programs..)
Go to http://www.nliteos.com/ download the program (RC3 seems to have a problem so you may need to go for RC1) and have fun
To change the profiles path you must select "tweaks". it defaults to %SystemDrive%\Documents and Settings but you can change it to d:\place_stuff_here or whatever you like.
You can also change the "windows" path, the "program files" (in the unattended setup option, may brake Windows 64bit), number of TCP/IP connections, set the services and, tweak the way Windows behaves during boot/shutdown, looks etc etc..
Or even better, kick out all unneeded drivers and use only those you need provided you have them handy.
Use at your own risk, change to much or set wrong option and you can really wreck your old installation. But with this nice util it is possible to have several Windows installs on the same drive.
If you want to change it on your current install I can try to find out if it is possible but I doubt it's easy and that it will work without any problems.