I'm pretty sure you need an activated (copy of at lease 2.54 = this is not accurate, I realized now that this was an issue with OPC not supporting SP2 on XP under 2.52.. apologies.) to use tags in an OPC fashion. If that's what you referring to. I just went through this very situation a few weeks ago and my local Rockwell guy was awesome about getting me set up. You do need activation though!
are you trying to use this just for experimenting/learning purposes? ... if so, you might want to do a search for Kepware ... they have (or at least "had") a free demo version that would run for a couple of hours of "playtime" ...
DISCLAIMER: I'm going by memory here - but I think that I've got this right ...