What software and hardware would you recommend for cloneing (cloning?)
the computer hard drive? I use Windows XP on a desktop. I do back up
files occasionally on a USB external hard drive, but have never made a
bootable clone.

I have used HD Clone with great results:

http://www.miray.de/products/sat.hdclone.html

It looks like they have made many improvements since I last used it, so I
might have to check it out again.

I normally use Macrium Reflect for day to day image backups.

Anthony