The Steam executable (Steam.exe on Windows) accepts a number of command line parameters; you could try taking advantage of those.

Specifically, this one, which starts Steam and immediately attempts to log in with the given credentials:

-login %u %p

This logs into Steam with the specified Username and Password combination. Replace %u with the username, and %p with the password you want to login with (Steam must be off for this to work).

And, since this doesn't work while Steam is running:

-shutdown

This shuts down (exits) Steam.

You can use Windows 7's Startup folder to open Steam to the right account when you log in.

What you would want to do is open your Start Menu. There should be a folder called Startup.

Every time you log in, Windows will automatically start all the programs in this folder. So, we can put a Steam shortcut with the %u and %p command-line options.

How to do it?

This presumes your Steam executable is in C:\Program Files (x86)\Steam\Steam.exe
You'll have to repeat these steps for every account you want to do this to.

1. Open C:\Users\[PC Username]\AppData\Roaming\Microsoft\Windows\Start Menu\Programs.
2. Right click and create a new folder. Call it Startup (Capital S).
3. Now, go into that folder and right click. Go to New > Create New Shortcut.
4. Now enter "C:\Program Files (x86)\Steam\Steam.exe" -login [username] [password].
5. Click "Next" and customize the name to your liking (or just leave it as "Steam").

Then repeat on your husband account, but this time, replace [username] and [password] with his username and password, and replace the [PC Username] with your husband's computer username.

You MUST be an administrator of the computer to do this:
Alternatively, if you both are logged in (ie. You're using as the active user, but he's logged in, but just stasis; not using; Switched users back to the login screen).

You'll have to manually stop the Steam process.

1. Press Win-R and type taskmgr (or press Ctrl-Alt-Delete and click "Start Task Manager")
2. If Task Manager is not already in administrator mode, click the "Show all Processes" button. If a prompt for admin password or administrator access comes up (UAC; User Access Control), just allow it.
3. Task Manager should close and reopen with that button gone and more items in the list.
4. Look for "Steam.exe" in that list. If it doesn't appear, it's most likely that you have not run Task Manager in administrator mode.
5. Select it and click "End Process"
6. Click "Ok" to close it.
7. NOW you can open Steam, but you may have to log him out first.

Alternatively, you can do it with one command: Win-R and enter taskkill /f /im steam.exe. Make sure taskkill.exe is run with administrative privileges as it needs to be able to close programs that are run by a different user.

Steam opening in another Windows User account | No. 7892142's comment
I'm at work right now, I'll try to come up with something up when I get home. (And can test it properly on my own Steam install, I wouldn't want to accidently run it as Admin and open a whole new can of worms.) -- For clarification: The script should kill your husband's Steam and run it with the %u %p command for you?

Instead of creating a shortcut in the "Startup" folder, you could create a batch (.bat) script that kills Steam and reopens it.

taskkill /f /im steam.exe
"C:\Program Files (x86)\Steam\Steam.exe" -login [username] [password]

.bat scripts can be edited using notepad.
(Right click the .bat file and go to Open With... > Notepad)
Say 'OK' to any prompts that come up. It should then open in notepad, or any text editor you have chosen.

Save your batch file in the same folder as the shortcut. You may remove the shortcut if you need.