Vanngile,
It's common to install the Windows and Program Files folders on the c:, and it's common also to install all programs in that one Program Files folder. But both are only a convention, and not a necessity. And there's no single rule for the location of data, nor any need for such. Having your data on a separate internal disk comes in handy if you make a habit for formatting and reinstalling Windows XP. But hardly anybody does so.
An external hard disk is a good way of taking data with you. But I should give a serious warning, and that it is absolutely a very bad idea to have your data on a such an external hard disk ONLY. The chance of losing it is just much too big, especially if you happen to format it FAT32. I would use a removable hard disk only for transport and backup, not as a replacement for an internal hard disk when travelling.
You can use an external hard disk to transport the setup-programs of your applications, but even if you install them to a removable, it's far from sure you can run it on another machine without reinstalling. And you might run into license and activation problems with some of them if you install them on another PC.
Hope this helps.
Kees