It sounds to me as if the photos were scanned at a very high resolution and then saved as raw bitmap (*.bmp) files rather than compressed into jpeg (*.jpg) form.
Most image manipulation software can do the job you require. Irfanview is free and easy to use:
http://www.irfanview.com/
Start Irfanview and use File > Open (or just click the 'Open folder icon). Navigate to the first picture and double-click the file name.
When the image is displayed, go to Image > Resize/Resample
Select an approximate size from the options on the right and click 'OK'.
Go to File > Save as. Navigate to the folder where you want the smaller image files to go.
Ensure that the 'Save as type' field shows 'JPG - JPEG files'
If you want, move the slider from 100 to, say, 80 to increase the compression. (You probably don't really need to do that).
Click 'Save'.
Assuming that your original image files are all in the same folder, pressing the space bar (or Page Down) will load the next file and you can repeat the process.
Chris
PS: If you've got lots of files to process, there are several free programs available for batch processing at www.download.com but it might be better to stick with a one-by-one technique because batch-processing software might attempt to load all of the files simultaneously, resulting in your PC crashing.