When I upgraded my old XP machine to a brand new Win 7 64-bit, I managed to install and run nearly 85% of my old programs / games. You won't get much issues as you think. If an older program will not run you can just run in compatibility mode.
If you are talking about really old games (80's and 90's), you'll need DosBox which is an emulator to help run older games (because on your new machine these games will be super fast).
Hope this helps.