Helping people with computers... one answer at a time.
Because of the files an installed application usually leaves behind, it's prohibitively difficult move it to another drive.
I have installed a second HD (200Gb). Can I get my apps from one drive to the other without reinstalling all of them? Also, can I leave XP on the primary drive but move the "documents and settings" folder to the new drive without upsetting XP?
The short, pragmatic answer to both your questions is no.
Let me explain why...
When an application installs, it leaves pointers to all sorts of files and such that it cares about in various places including the system registry and quite possibly files and locations known only to that application. There's no way to go in and change all those settings and know that you've gotten them all. So the "right" way to move an application is to uninstall and re-install it to the new location.
Be aware that some applications will still install some components to "c:\program files" even if you are installing the full application somewhere else.
As for moving Windows' "c:\documents and settings", technically I believe there is a registry setting that defines its location. However, I would be extremely reluctant to change that. My concern is that I fully expect some applications to ignore that setting and always write to "c:\documents and settings" anyway. My suggestion here is to have your video or other application simply use directories that you create on your new drive, thus ignoring the whole "documents and settings" thing all together.