Welcome, I would scrap Spybot Search and Destroy, it's becoming more obsolete as other vendors like Malwarebytes take over.
Even though Avast has an Auto-Sandbox, it's not the greatest when running downloading and executing unknown programs from the web and emails etc. Remember you can turn on PUP-detection for Avast in each of the Shield settings, I think it's under Sensitivity.
Optionally, the browsers. IE10 for Windows 7 and Firefox both require an additional download for Adobe Flash Player, which either you let it run in the background to update, or you manual updates. A few extra resources running in the background. Remember to uninstall Java or other plugins (ie. Silverlight etc) from the Control Panel, if not in use or required. Silverlight updates can be hidden from Windows Updates.
In IE10, I prefer to use the TPL for Adblocking here:
http://fanboy.co.nz/ie.html
To add to McLovin's post, HTTPS Everywhere is available for Chrome and other browsers.
https://www.eff.org/https-everywhere
Last thing, I recommend AdBlock over the more popular ABP. AdBlock doesn't have a pre-defined white-list of Ads.
http://getadblock.com (redirects)