XP should be fine on the 400 MHz, as long as you've got enough (minimum 256 MB) RAM. We're still pushing a couple of Pentium II 350's w/384 MB of RAM, and they're fine for email, Word and basic browsing. Heck, we've got around 100 Pentium II 233's w/128-256 MB of RAM running Windows 2000... Get lots of complaints about them being slow, but you have to give the hardware credit for running reliably for 12 years.
