I'm starting to play old DOS games again and hope I can use ScrummVM for them on my Win 7 PC. I still have Win XP, Win 98 and Win 95, but the latter two are in a closet and too much trouble to unhook Win XP and hook them up.

If I can't use ScrummVM on Win 7, I assume I can download it and copy it onto Win XP? If the answer is I can't use ScrummVM, how difficult is DOS box to install and use?

ScummVM will run on any version of Windows, including XP, 7, 8, 10.
However it does not support all DOS games.

Here is a list of games supported by ScummVM
https://www.scummvm.org/compatibility/2.1.1/

Note that some are Windows games and some are DOS games.
For example Myst and Starship Titanic are Windows games and are supported by ScummVM.

ScummVM works by substituting the original game exe with a ScummVM exe, which is why saved games don't work between ScummVM and original DOS/Windows versions of the games.

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DOSBox is not hard as long as you are familiar with running games in DOS. If you're not, it can be something of a challenge. DOSBox is like having a little DOS machine running on your computer. It's configured so you don't have to worry so much about memory restrictions that people had with DOS games on a DOS computer. You do have to worry about mounting drives, but it's really not that hard as long as you're familiar with DOS and running things from the command line. There is a basic Tutorial for DOSBox here
https://www.dosbox.com/wiki/Basic_Setup_and_Installation_of_DosBox

The Sierrahelp installers for Sierra games use DOSBox and preconfigure it for you.
http://sierrahelp.com/Patches-Updates/NewSierraInstallers.html
If you're mainly interested in Sierra's DOS games, that might be easier than configuring the games yourself.

DOSBox has a compatibility list with tips for running different DOS games at
https://www.dosbox.com/comp_list.php?letter=a

Some people find it more convenient to use one of the Front Ends for DOSBox instead of configuring manually by editing files.

Jenny100, thanks for the info. Turns out the game I was going to start with is on ScummVM's list. It's been decades since I played in DOS, so I'll save that until I get a game that doesn't work on Scumm. Appreciate the links to DOSBox, I've bookmarked them for the future.