Guinea pigs really don't do well in the cold. It is a popular myth that they are hardy but they can get cold very easily and can get respiratory problems.
If you cannot bring them in then please make sure they have plenty of bedding and make sure that the bedding is not damp. In addition, their cage will need to be protected from the weather.
Another worry is their water bottles freezing up.
news.bbc.co.uk/cbbcnews/hi/newsid_9240000/newsid_9245300/9245344.stm?ls
For water bottles
www.the-rabbit-hutch-shop.com/rabbit-hutch-water-bottle-insulator-bottle-snug.html
For hutches
www.the-rabbit-hutch-shop.com/hutch-hugger-rabbit-hutch-cover-94-l-x-49-d-x-49-t.html
www.amazon.co.uk/Snugglesafe-Microwaveable-Heat-Pad-Cover/dp/B001A36EUG/ref=sr_1_14?ie=UTF8&qid=1291218852&sr=8-14