Most programs will work. If a program uses some processes, though (some Norton free services, for example), it will tell you that the program is made for a 32-bit version of Windows and won't work on your 64-bit copy.
In case a file doesn't work in 64-bit, you can always use the handy Compatibility tools pre-installed on your computer; if it's something meant to change system-level files, though, I recommend against it, because it leads to all kinds of nasty errors.
You'll probably never run into any issue of that sort, though. Probably.
Edited by isaved007, 29 April 2010 - 10:19 PM.
Disclaimer: I reserve the right to bash all products and services offered by Apple, partly because I don't like them, and partly because I fail to understand the demographic that buys Apple's products. Fell free to bash Microsoft products and services in my face; I like hearing how wrong I am about everything.