When you replace a mobo you pretty much have to reformat, although in some cases I have heard you do not. But replacing the mobo is basically building a new machine due to the fact that all the parts will act differently than before. So what I am saying is that you need to reformat your Pata drive....if you want to keep all your information from that drive you will need to either transfer it to another computer, or buy another hard drive and use your pata drive as a secondary.
Also you cannot reformat with the windows cd from dell, you will need a vendor copy of windows xp (which you can get off ebay) or windows vista which is in every retailer now.
You should have asked us before upgrading that dell, we could have assisted you with buying the best "bang for your buck" parts. But hopefully we can get this rig up and running