Dear Lifehacker, When we're travelling or away from her apartment, my girlfriend borrows my laptop. She uses a different browser, different work applications, and ends up emailing herself files. What's the best way to set up a second account for her convenience? Signed, Laptop Loaner
Dear Laptop Loaner,
First off, you're very considerate to set up a whole workspace for your significant other, and it's probably going to help you avoid at least a few tense moment when she sees your anxious face as she logs into and out of services, changes trackpad settings, and asks why her IM settings are all messed up.
If it was truly just some browsing the non-owner needed, you could set up an Incognito Mode shortcut for Chrome, which would launch a browser window that lets her log into her Gmail while you stay logged into yours.
But, as you stated, there's work to be done, files to be opened, and settings to be tweaked.
Go ahead and head to your user settings on your machine. For Windows and Mac users, it's fairly straightforward to create a new account, give it a password, and make it a "standard" user, rather than an administrator. On a Mac, you can also make the call as to whether the system boots up to a user account choosing screen, or always logs into your account, where you can then log out to reach your secondary account (actually, don't tell your girlfriend it's a "secondary" account—seems like a deep conversation waiting to happen).
For the files, do what I did—set her up with a Dropbox account, if she doesn't already have one. Using symlinks to sync folders outside the My Dropbox folder, you can have Dropbox keep the contents of her desktop folder synced directly to her user account's Desktop folder—usually C:UsersGirlfriendNameDesktop on Windows, or /Users/GirlfriendName/Desktop on a Mac. That's a good option for the Desktop, Documents and other folders if they're generally not stuffed full of big files. If you'd like to make it a bit more selective, you can create a folder on her own laptop named something like "Sync to Boyfriend Laptop," and she can choose what files end up available on your own system. Generally, though, you'll want to go the automatic route, so you can look like Mr. Magic when the file she thinks she forgot to send herself magically appears on her borrowed desktop.
While you're getting all wizardly with the Dropbox symlinking, see if you can't switch your sweetie over to Pidgin or Adium for IM chatting. They're fairly powerful multi-protocol IM clients, and you can sync their profiles across computers, keeping one's preferred settings and Buddy List organisation intact. That sounds like a small thing, but it's the kind of small thing that grates on someone until it's fixed.
As for necessary work applications, go ahead and install them while you're logged into her account for the first time, because you're the one with the Adminstrator access/password. Don't just click-click-click, though—look for the common option to install the application for "Everyone on this computer" or "Just this user," which is easy to ignore when you're the only user account on the system. Pick "Just this user" or "This user only," unless you have some use for that application, too. If you're using a Mac and she's a PC worker, I've run through both the elegant-but-paid and rough-but-free solutions to running that one Windows app (she) needs on a Mac.
Finally, since so much of the computer is in a browser these days, set up her browser to sync itself between her computer and your user account for her. If she's a Firefox fan, the Weave add-on can keep everything in sync—bookmarks, passwords, settings, even the last set of tabs she had open. It's how my own wife works between computers, and it's scarily seamless. On other browsers, Xmarks can handle the bookmark syncing (though Chrome and Opera have their own built-in bookmark syncing), and LastPass has you covered on passwords.
Aside from desktop backgrounds and data storage allowances (a whole other post in itself), that should get you started with a harmonious way to share your laptop in loving kindness, and not completely bug out when you find your desktop background changed upon logging in.
P.S. Anyone with their own experience sharing their system is welcome to share their tips, and stories, in the comments.