I just installed Windows 7 on an 8+ year old machine I had laying around my house and I was surprised to see that it runs as well as it does. In fact, it runs better than the XP installation I had installed on the old PC before upgrading it to Windows 7. To be fair, the Windows XP installation was a few years old and probably suffered from disk fragmentation and memory being consumed by the random applications and services that had been installed.
Hardware that I had difficulty with in Windows XP was automatically recognized and had the proper drivers installed. Even the cheap-o USB wireless stick I bought a few months back was recognized during the installation so I could setup my wireless network before even booting into the full operating system. On my windows XP installation, I had a bunch of problems getting the USB stick to work and had to get drivers directly from the manufacturer. With 7, everything was automatically recognized and installed.