if you are using word, go to tools & then pick envelopes & labels. when you buy the invitations, the directions sheet will tell you which template to use. oh, pick labels section & then options. once the template is chose, pick 'create new document' & then you'll be able to insert pics & text

Do you have Microsoft Works? click on start (Lower left) then All Programs, and see if it's there..click on it.

Depends I suppose.

Obviously you will need a printer.
This requirement alone has several options.
Do you want color? High quality prints? usb printer?
If its a usb printer your PC has to have a usb port. If not then you need a LPT type printer.

Will need paper:
What color? Thickness? Photo quality paper?

And depending on if you want to create your own invite cards you will need a program to do that.
Your PC would be best to have windows XP.
As for software you can try to search for it.

If you have microsoft word you could try using a template to make a simple card that can be folder after it is printed (have done this before myself) this may not be too fancy though.

if you dont have word:
search on google.com or sourceforge.net