You should have:
1. A software firewall (NOT the Windows version).
2. An Antivirus (AvgFree, for example. I would avoid Norton) providing real-time protection.
3. Several anti-spyware applications, at the minimum Ad-AwareSE and Spybot Search and Destroy. Others are optional, but useful.
4. Firefox (or Opera) for safer, faster browsing. Optionally, Thunderbird to replace Outlook Express.
5. OpenOffice. org. This is an open source (and free) replacement for Windows Office, with great functionality and it interfaces well with MS Office file types.
The next set is useful in web surfing and downloading, and file creation:
6. Adobe Acrobat Reader (or a slimer equivalent depending on your use).
7. Probably Quicktime and (alas!) Realplayer.
9. CD burning software (e.g. Nero)
Anything else would be the result of your own computing activities. IM applications, software required to run your peripherals, picture or music editing software, etc.
I would direct your attention to a pinned post here at BC called "Slow Computer Checklist" which provides a quick guide to the steps you should take on a regular basis to keep your computer running at its best:http://www.bleepingcomputer.com/forums/t/44690/slow-computer/