Hey Jantey! Winters is the nost amazing time to visit Manali and Parvati valley. Go in January mid, you will find snow (fingers crossed). Yes, its possible to go Manali and Kasol. Both are situated at a gap of 3 hrs (approx.). You can take a local bus till Bhuntar and then a cab till Kasol. Or staright away take a cab from Manali market towards Kasol.
In winter, most of the roads to Kasol are inhospitable due to heavy and frequent snowfall, and if not this, then cold and dry winter winds make it a little difficult to explore the valley.
Winter season is considered to be the best time to visit Manali if you like the cold and January is best to enjoy the chilly pleasure of fresh snowfall . The temperature drops down below zero degrees Celsius.