You will only be able to run programs compatible with Win7. Some are not. Check the publisher's websites for info. Also some hardware, most notably older printers, are not supported under Win7. The manufacturers want you to buy a new one with the new chipped ink-tanks. There are some workarounds for older printers but you tend to lose driver functionality at best as they rely on the native windows drivers.
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So, for example, Starcraft 2, a game, is compatible with windows xp, vista, and 7. If I back up the files on an external hard drive, would I have to re download them once I do a complete Windows 7 install?