Do you have a 64 bit CPU? If not, the question is moot. To find more info than you ever wanted to know, click on Start/Accessories/System Tools/System Information (you might need to click on "All Programs" after "Start" if Accessories isn't on your startup screen). If your processor is shown as x86, it's a 32 bit processor. Although MickeySoft did release one 64 bit version of XP, I don't believe XP Media Center was involved.
I've been running Win7 64 bit since I bought a new computer with it installed last fall. I've got mostly praises for it. I've also installed Windows XP Virtual Machine to run some of my old 16 bit programs. So far, I haven't found any 32 bit programs that won't work, although I've had to open the properties screen and change compatibility options for a few of them.
Peripherals may be more of an issue than programs with a 64 bit system. If you've got older peripherals the manufacturers may no longer be updating the drivers for them and you'll be out of luck, except you still may be able to use them with Virtual Machine. I have only a ten year old HP printer attached to my Win7 machine. It works fine, but the automatic duplexer I added on to it isn't even recognized. No biggie for me,since I rarely used it anyway. Manual duplexing was far faster for any big job.