Ben,
Ok, sorry, I wasn't sure how in-depth you wanted to go. If you are worried about them changing preferences like the desktop background etc., those types of things will only be reflected in the guest account (or whatever account you create), called a profile. As long as they don't log into your account, nothing will effect your profile.
Here's what I would do. If you simply want one account for the whole family to use, go to your control panel (Windows key + X > Control Panel > User Accounts) create an account called Family (or whatever) and make sure it is of type "Standard". Now, click on 'Change User Account Control Settings" and slide the bar to the highest setting (Always Notify) - this will force an account control prompt for a password (your admin account password, so make sure they don't have it) anytime they try to install, download, or make any changes to the computer.
Now, this might be overkill, but, first log into the account that you just created. Go into Internet Explorer options. I use Firefox, so I'm working off memory here, but if you go through the options, there should be a toggle for "Delete History on IE exit" make sure it's on. Then go to the Security tab. Drag the slider to High or Medium High. Go to the Privacy tab and do the same. Also look for the pop-up blocker options and make sure this is on as well.
I believe that the User Account control setting on high should be enough though. You can try it out just to make sure. Log into your newly created account and browse around the Internet, try to download something and see what happens. You should be prompted for a password (which they won't have) and therefore will be prevented from downloading/installing anything dangerous.
On a side note. You could log into the new account and install Chrome or Firefox. 99% of the malware out there only effects IE and after I got rid of it, I've never had one problem on any of my machines, incl., my kid's and wife's - who are not careful at all - and this is without any antivirus/malware protection/defender/firewall whatsoever. The only thing we run in the browser is "Adblock".
If I'm off base as to your needs, let me know.