In the winter you can ride the following
Bearbrook
pawtuckaway
rockingham trail
LMFT
Warren
Jericho
Newport/claremont
There is actually more trails open for atvs in the winter than in the summer in NH. NH sucks for summer riding, but winter riding it's pretty good. Go over to ATValliance.com the new england forum, all nh riders over there an all the info you'll need.