Face the truth, although there is a high level of compatibility between Windows 7 and XP there are things that will only work on XP, just using Windows 7 64-bit or Windows 7 32-bit already makes a big difference. And using an ATI or Nvidia graphics card also makes a difference, such as the way to scale the image of games created for 4.3 and not intended for widescreen, in my experience, ATI does this better than Nvidia, but Nvidia can be More compatible at the Driver level, but I don't know which will be the latest version that is more compatible with DX9, because the most modern Nvidia I have is an 8800GT.
If this didn't happen, many of us wouldn't have so many computers from different eras and with very different hardware.
From my experience, once you overcome the problems with anti-copy systems, the big difference is in the hardware you use, not so much in whether you use Windows 7 or XP. And Using 32-bit Windows 7 greatly increases compatibility, since 16-bit applications still work without problems while in 64-bit Windows 7 it is not so easy, and that causes you to not even be able to use the original installer of some games.
But I think at least 90% of the games released for XP will work without any problem on Windows 7.
Like I said, on Windows 7 32-bit, I haven't run into any serious problems yet.