The simplest answer: Linux liveCD, when you're doing work
offline (the keyword being "offline" here) turn off or unplug your modem. Inconvenient yes, but you're not going to get any more secure than that. I only mention a Linux LiveCD because when you turn off your machine any work you have done on it is lost forever since it is all loaded up in RAM. Any work you wish to save, use an external hard disk or something similar. Secure said hard disk with Truecrypt (
http://www.truecrypt.org/) Truecrypt is open source, so you don't have to worry if there's backdoors or Trojans inside of it.
I wouldn't go with a huge terabyte hard disk as it's inefficient and if you need to take the information with you in case of an emergency you'd have to rip apart the PC case to take it out. Assuming your work is not high quality video editing or something like that, then I see no need for a really huge hard drive.
Alternatively, I think there are PC cases out there with lock and key and easy removable hard drive bays, I'm sure that Google will turn up something on those.
If you're going the Linux LiveCD route, obtain hardware that is fully supported under Linux. What's great about most LiveCDs is they will tell you if your PC will be able to run linux or not.
Hope this helps, and good luck.