I highly recommend running the Windows 7 Upgrade Adviser
before attempting an upgrade...it will alert you to any potential conflicts.
Older versions of iTunes should be uninstalled before upgrading to Windows 7, but the new version (9.0.1) installs and runs just fine. I've also seen some versions of McAfee causing problems during the upgrade process...I'd suggest uninstalling McAfee before upgrading, as well. Norton 360 apparently also has problems with Windows 7.
I have found very few programs that won't run under Windows 7 64 bit. Some older hardware won't run as the manufacturers do not plan on providing drivers. Again, the Upgrade Adviser will alert you to any hardware that won't work with Windows 7. 64 bit Windows 7 is a quantum leap ahead of Windows XP 64 bit and I really suspect that this will be Microsoft's last 32 bit operating system offering...of course, this is pure speculation on my part, but it sure seems to be the way things are headed.