Well, depending on which OS you are moving to (W2k, etc.), you should just be able to connect your old hard drive and boot from your new drive when the OS setup is complete. After you are in the OS, you should be able to use Windows Explorer to copy all the files that you want over. You can copy program directories over, but the associated Registry entries for the program will not get copied in the process. Some programs do not have a problem with this and will recreate the necessary entries the first time you run them under your fresh OS install. Other programs, quite possibly Word, Access, etc, will have a problem if you just copy them over and usually requires a clean install. If its just data files that you want to copy, you should be fine.
Now, there is a big problem that affects W2k that I ran into a couple of weeks ago at a customer site. I had just installed a clean W2k, along with all device drivers, etc. They wanted files copied from the old hard drive which had Windows 98 on it. Well, I copied all of the files over from the old drive and shut the machine down so I could take the old drive off. I reboot into W2k and I cannot log into the system because it said the swap file was too small or did not exist. What happened was the W2k somehow set some of the swap file up on the Win98 drive.
Long story short, I ran fdisk /mbr from a dos disk, and rebooted into W2k just fine. That will reset the partition/drive information that W2k sees. I'm not sure if this problem would effect either of the XP's, but its a possibility.
Hope this helps!
Matt