If you can download and install Quantum GIS, then you're most of the way there.
Here's how to do it:
1) Download and install Quantum GIS (Download - Quantum GIS Wiki choosing 'Windows Standalone Installer').
2) Double-click qGIS Setup file to install. Follow instructions.
3) Download the NBN Watsonan VC files (Resources - Mapping - National Biodiversity Network).
nb There are lots of options, choose 'Individual County Files Polylines folder', unless you want the dataset for the whole of Great Britain as one file, which you probably won't (if you do, it'll be a 216 Mb KML (Google Earth) file once converted).
4) Right-click downloaded Watsonian VC .zip folder. Choose 'Extract all'.
5) You'll find another zipped folder. Extract this too, preferably to the Desktop. (May take a while, especially on an old pc, make a cup of tea).
5) Open folder (on Desktop) once extracted.
6) Find your county's Shapefile (in ESRI folder) e.g. Berwickshire_polyline.shp.
7) Copy this file somewhere memorable (to the Desktop?)
8) Open your new qGIS program (link should be on Desktop)
9) Go to: Layer --> Add Vector Layer in qGIS program (on tool-bar at top of qGIS window)
10) Click 'Browse' button and choose the .shp file you've copied to the Desktop. Make sure file type is '[OGR] ESRI Shapefiles'. Click 'Open'.
11) Now you have your boundary in qGIS. We need to export it. First you'll need to select this 'layer'.
12) Go to: View --> Select Features. This will give you a cursor with which you can drag a large box over the entire VC boundary. If you do this successfully the VC boundary line will change colour.
13) Go to: Layer --> Save Selection as vector file. Fill in the following: Format = Keyhole Markup Language; Save as = 'Insert appropriate file name here', Browse to an appropriate folder to save in.
14) For CRS (Coordinate Ref. System) click 'Browse'. Search using 'Name' for 'OSGB 1936 / British National Grid'. Select this, and click ok. Then click 'ok' on the 'Save vector layer as...' box.
15) Find your new kml file and open it in Google Earth. Marvel at your GIS prowess.