How do I use try .. except or if ...else to validate user input?

I'd suggest a combination:)

while True:
    value = raw_input('Value between 0 and 100:')
    try:
       value = int(value)
    except ValueError:
       print 'Valid number, please'
       continue
    if 0 <= value <= 100:
       break
    else:
       print 'Valid range, please: 0-100'

Hope it helps.

if/else is probably more appropriate here, since any exceptions raised would be ones you threw yourself (and you'd still have to handle them).