If you no longer need a program on your computer, or if it is conflicting with other programs on your system, it's a good idea to uninstall it. Most of the time, the program's uninstaller will be adequate for this. You can find that by looking for the installer file, and then selecting "uninstall." This should remove all of the files associated with that program. There may be many other programs that wound up getting installed on your system that you're not even thinking about, and are just taking up space or resources.
Leo says it's very difficult to move an app to another computer. Not only is it difficult just to move those programs, he's also moving them from 32bit to 64bit which in some cases is just not possible. Windows Vista has the capability to move programs built-in, but Leo doesn't recommend doing this at all. He really should buy new apps or install them from their original disks. Another option is to clone the drive from the old computer, and dual boot Windows XP and Windows 7 on the new computer.