I've seen a few posts where people are frustrated by the fact that XP excercises control over com and serial ports making it difficult or impossible to use older dos based programmes. I have found an app which allows control of both serial and parallel ports called
DirectIO. I have a copy which was purchased from JED micro here in Australia. I haven't tested it yet but I was assured by JED that it works very well. I will test some time in the near future and report back for anyone interested.