That's true it depends upon which server you want to play on. The simple answer is to have EQ installed once, then copy it to another folder.
Example: (my folder structure)
C:\games\UF Client1 - the normal one I play that is a base install
C:\games\UF Client2 - the 2nd account I play with above, base also
C:\games\UF THF1 - special one for The Hidden Forest server with special files to play on that server
It can get a little complicated if you have two clients that need modified files for a particular server. Just have to rememberto update both!
Also, when I got a newer computer, I experimented with the install. What I did was copy over the client folder to a backup drive, the just copied that to the new computer. So it doesn't really require install disks if you copy the entire folder of the client. However, to unintall the program requires you to just delete the folder. No add/remove programs in that case.
As an explanation why to use separate clients for another account is for more stability. I know some people who use one client to play multiple accounts, but have seen some instability.
Another thing I do is us the client in windowed mode or use the free WinEQ lite (or other programs that this community may know.) Easier to look up stuff or pay attention to you tradeskill guide.
Sorry if this long, I used to tech support on the phone and emails, so I like being thorough.
Good luck!!