If you are using a Windows operating System (I assume you are)
Then download JKDefrag. It will defrag your computer making it a bit faster.
http://download397.mediafire.com/c5eeid73y8sg/13tt71q3qerwyhx/JkDefrag.exe
Before you run JKDefrag, you should also go to your Start Menu and click Run and type "compact.exe /u /i /s" (without quotes) and press Enter. What this does is it uncompresses files that have been compacted so they can be defraged. Make sure you run the defrag once a month, to avoid your computer from being fragmented and slow.
For your anti virus. I recommend AVAST anti virus free. It scans things other anti viruses won't like bit torrent, intant messengers etc. Also you can register it.
You should also download CCleaner which cleans up old temp file and other unnecessary files on your computer. It also can clean your registry to, getting rid of what is left behind from installed programs. You should run this at least once a week, depending on how often you use your Internet.
http://www.piriform.com/ccleaner
For your web browser, I highly recommend Mozilla Firefox. It is the most secure browser out there and fast too. It is the only browser I ever use and wouldn't use anything else. You can install addons too. I recommend you download IE Tab for access IE only websites like Windows Updates.
http://www.mozilla.com/en-US/firefox/
https://addons.mozilla.org/en-US/firefox/addon/ie-tab-2-ff-36/
And last thing, if you use an external mail client look Microsoft Outlook or Outlook Express. I highly recommend you switch to Mozilla Thunderbird. It is a lot more secure and more up to date.
These programs are something every Windows user should have.