Hey, there are several places to rent high quality road bikes in Boulder, only 20 minutes from Longmont. I'd start with University Bikes.
http://ubikes.com/articles/bike-rentals-pg432.htm
Trek, Bicycle Village, and Boulder Bikesmith also rent bikes.
Road conditions are good right at the moment, but it's highly variable in winter. It was too snowy to ride 3 days ago, and it's going to snow again on Sunday. If it snows and stays cold, it can be a problem, but if it's sunny for even a couple of days, the roads melt and you're good to go. They do sand the shoulders, but it's manageable. Be prepared to ride in about 35-40 degree temps, or even colder if you don't have a lot of flexibility about which day(s) you can ride. There are lots of nice roads between Longmont and Boulder, and also north of Longmont - if you tell me more about where you are in Longmont and how long of a ride you want to do, I could make some suggestions.