Snow tires for H Plus Son SL42?
For a complete knowledge base on fitting tires for different rim-sizes, see here: http://sheldonbrown.com/tire-sizing.html
and scroll down to 'Width Considerations'.
30 or 33mm tires will fit your rims but they'll probably need a lot of pressure which might just be counter-productive on snow. On the other hand you should consider another possible issue: 30 or 33mm tires might not fit into the front fork or between the rear stays.
If they do, you might want to buy a second pair of wider wheels and use the wider tires on these. It will keep your 'Summer' wheels nice and shiny.