Actually, yo can do it all in ArcMap, without using any addtl extensions. First create a table of your office locations with the street address, city, state and ZIP in separate fileds. You can have other fields like office name, or whatever you want too. If you're using 9.2 it will take an excel file, otherwise create it as a DBF, or in MS ACCESS (or a geodatabase - though not as easy as Ecel or Access for a small list like you have). Then add that table using the "add data" button. Nest use tools>geocode, sleect one of the geocoding tools that comes with arcmap. It uses the Teleatlas / StreetMap USA layer to geocode. It's pretty accurate. Alternately, you do a find>address for each and get the lat & lon from that, add that you your table, then bring the table in as an X / Y layer.
To get your buffers, use the proximity tools in arctoolbox and you can create a buffer in whatever units you like (feet, miles, etc) for the locations you added above.
Geographer, Buffalo, NY