Yes , you will need a MIDI interface . Most of the low cost ones will be a USB midi interface .
Also , do you have an audio card ?
If you have a Soundblaster card , that will not cut it . Soundblaster is a multimedia card , you will have latency ( stuttering ) in Reason and other software synths .
A great cheap audio PCI card for PC is the AudioPhile 24/96 . It also has MIDI , so you will get both for less than $200 . You will get 24 bit audio at 96khz , which is pro audio quality recording for just a few clams .
Roland Juno-D , AudioPhile 24/96 , free software synths - now you are talking !