if you have an external HDD plug it in. drag said contents you dont want to lose across. plug into new pc. drag across again. profit.
if you make notes on Stars then remember to take your username.notes file from
C:\Users\Supernova\AppData\Local\PokerStars.UK\notes.Supernova.xml (most likely location)
if you cant see appdata already, go to start and type 'folder options' click on top hit, then go to View tab and look down to advanced at the circular buttons under 'hidden files and folders' and turn on Show hidden files/folders/drives.
there are always programs you forget save stuff to random places outside of documents/pictures folders etc.
if you already have PT4 with a database, then go to here and follow the instructions to make sure that is saved and copied to the HDD when moving stuff
https://www.pokertracker.com/guides/PT4/tutorials/backup-and-restore