Windows 7 or XP?

City of Heroes uses OpenGL as it's rendering API. Providing everything works properly, you'll get the same visual image no matter what Operating System you use.

Decisions about what OS to use then are a moot point with CoH as a reference.

Other factors such as stability, security, performance, support, and price should be the basis upon which you choose an operating system.

Yes it will run properly but which one takes up the least amount of operating space to leave more room for the game to go at it? Your answer is moot.

In that case you should go XP. Windows 7 adds a lot of new features but it is more resource intensive. One thing to watch for is the memory limits. If you have more than 2Gb of RAM you need something other than the basic version of either operating system and with more than 4Gbs you need the 64-bit version.

If you are referring to physical drive space:
Windows Xp SP3 takes up: 3.8gigs
Windows 7 takes up: 11.8gigs

If you are referring to Operating System overhead
Windows Xp Sp3 is Windows NT5: it's relatively light-weight
Windows 7 is just Vista with the huge winsxs setup needed for NT5 backwards compatibility ripped out. Lightweight and svelte are not in Win7's list of features.

I think the most telling aspect is that for the current x86 netbook market of Atom processors, Microsoft willingly sells and supports Windows XP because it will run on weaker hardware. Microsoft won't let vendors place NT6 operating systems on such low end systems because the performance is just so bad.

However, once you exceed a certain amount of base processing power and memory, the performance differences between NT6 and NT5 become meaningless. If your friend is indeed building you a super computer

say.. An AMD Phenom II X4 945 Deneb or higher, or a processor from Intel's I7 Lineup... with 4 gigs of ram... and a RadeonHD 5870...

You won't be able to tell the difference between which OS your computer is running.