Things to install after a clean format!
One of the first things I'd recommend is an anti-virus suite that also includes anti-spyware. A few of the free products recommended are: Microsoft Security Essentials Avast Free Comodo AV Free
Since no anti-virus product is 100% effective 100% of the time (if ther was such a thing we'd all be using it) it's also suggested that you have another on-demand scanner that can double check your computer in case something slipped by your primary anti-virus. Malwarebytes is highly recommended here. Malwarebytes
Any time you download software, especially free software, read the download info carefully. Companies may try to download other things like browsers, toolbars, etc that you may not want and that may cause problems for your computer.
As far as any other programs thaat you need to install, that really depends on how you use your computer. For example, doo you do a lot of photo editing? Do you need a full office suite? Rather than try to figure out eveything you need and install it right away, wait until you need something and then ask around for suggestions.
In the meantime, to help try to get back your computer's speed, take a look through this tutorial on how to optimize Windows 7. You may find that some of the suggestions like Disk Cleanup, Defragment, etc. Optimize Windows 7