If you can't or wont pay for a zip program then 7Zip is one of the best options. 

Its quick and easy to install plus the download itself is absolutely tiny compared to the hugely weighty Winzip - the only downside is that its so simple and basic to install that you do have to manually configure the file associations but even that is quick and easy (see below). 

The ONLY thing I can find to complain about with this program is it does add quite a lot of options to its right click menu, some of which may well be confusing for the inexperienced user and perhaps their website could be a little prettier if I'm being really picky :D

To set up the program for regular use you may need to associate it with the types of files you want to open. Run 7Zip (you will find it in your Program Files folder and/or in your Start menu). Go to the Tools menu and Select Options:-

 In the Options Panel, choose the System Tab. You can select all of them if you wish, personally I would suggest the 3 highlighted below will probably be sufficient for Sims downloading. Once you've made your selection click OK and close the program.

Once you've chosen, click OK and close the program or if you want to change the language, click on the Language tab and select your chosen language from the drop down box if it is available.

For installing Sims custom content, you can if you wish download your zip or rar files directly to the folder the files need to be in and then follow the right click, select 7Zip > Extract here to unzip the files into that folder.

You can unzip multiple files - simply drag a selection box around all the files you want to extract, again right click and choose 7Zip > Extract here or Extract files... to choose a different folder

http://www.7-zip.org/faq.html

If your question is not covered there or you have additional problems using 7Zip please contact their support service - we cannot provide technical support for 7Zip as we did not create the program.