You'll obviously need whatever apps you want for your daily life, such as an office suite, etc. In addition, get these two free spyware removers --
Ad-Aware and
Spybot Search & Destroy.
They're both free, and they both do the same things, but one sometimes catches problems the other misses. Spyware can be as malicious in its own way as viruses.
Download both, install them, and get the update files for both. Then, disconnect from the web and run them.
One other program I rely on is Norton System Works, especially WinDoctor, to keep my system running smoothly. It includes a year of Norton Anti-Virus, which you already have, so you'll be paying for your next subscription which you can use when your current one expires. You'll have to contact Norton (Symantec) to find out how that works.
System Works Pro also includes Norton Ghost, which can save your butt from almost anything.
Ghost can clone your hard drive. That is, it makes an exact,
running duplicate of your drive so, if your drive becomes corrupted or virus infected, you can Ghost back from your last good backup, and you're completely restored to where you were. If your drive dies, replace it with the Ghost copy, and it runs just like the original so you're out an inexpensive piece of hardware and the time it takes to replace the drive, but not your files or your setup.
To use it, you can buy another drive, preferably the same size as your current drive. You'll also want
a mobile rack. These are enclosures that mount in a 5.25" bay and hold a 3.5" drive. The link is just to show you what they are. You can buy them wherever you find them, locally.
Ghost runs from a floppy disk. To use it, you power down, plug in your backup drive, and boot from the floppy. When you're done, power down, and unplug the backup drive. There is no virus that can jump the air gap to an unplugged drive.
You'll probably get far more replies about your hardware questions by posting them in the General Hardware forum.