Speed work in the winter time is a little tricky. When running outside in subfreezing temperatures you never want to push too hard.
The key in winter speed work is to warm up appropriately. If you are able to bike inside, do it for about 15-30 minutes if not then run at a slower pace.
My speed workout of choice to do once a week is:
2 mile warmup
8 repetitions of 20 seconds each sprint (sub 5:30 pace)
10 second rest in-between reps
Cooldown for 1.5-2 miles