For antiviruses, I use
Microsoft Security Essentials. It's free, and does the job just as good (and often better) than paid antiviruses, as long as you have genuine Windows (I can confirm it also works with pirated Windows + Daz Loader). I do a weekly quick scan using both MSE and the free version of
Malwarebytes. I also use
Firefox with
NoScript and
AdBlockPlus. NoScript ensures that literally no scripts (JavaScript, Flash, redirects, popups...) will run unless you give them permission.
Just look at how much crap it blocks from loading on an ad filled website like Cracked. AdBlockPlus with a subscription blocks all ads from loading, on every web page. Not only does it clean up every page, but it blocks malicious ads too. Note that the Firefox version does not even allow the ads to download and plain removes the frames from the page, unlike the Chrome version which allows the ads to load then makes them disappear from the page.
Regularly run a defrag, Windows 7's (and Vista too, I think) is perfectly fine and helps a lot.
Disable some useless services that are running and not being used. The safe option is a good optimization, I personally use a mix of the safe and tweaked options.
Disable the unnecessary features that Windows has on by default. I only have .NET Framework enabled as i'm not sure if disabling that will uninstall it. You can get to this by going to the Computer folder, clicking 'Uninstall or change a program', and then clicking 'Turn Windows features on or off'. The screen before can also be used to quickly uninstall shit you don't need anymore.
Disable programs from running on startup. To do this, press Windows+R, type in msconfig, and hit enter. Go to the startup tab and disable any shit you don't need. Typically only your antivirus and anything related to drivers need to start on startup.
CCleaner can clean up the registry and get rid of a whole lot of unnecessary crap.
Another thing I disable immediately on a new install is UAC. It is completely useless and a pain in the arse.