If you just have a single HDD, and have a couple of partitions on it, you can put all your data on the 2nd partition and reformat the primary before u install the OS on the new computer. I just did it this last week.
Or you could just backup the data and nuke the whole drive. If you are thinking of "just plugging in" the HDD into the new computer and wait for it to start up and work normally, that might be somewhat problematic
.. Driver issues, windows xp activation issues, etc. etc. Remember that the s/w you install to talk to your hardware is specific to a particular set of h/w.