Any machine that runs vista will run XP. Someone wanted to sell you vista.
Like I said, I don't agree. And skuzzy said
Any desktop machine. Laptops,...not so much, unless the laptop manufacturer has specific driver support for XP.
The machine on which I was trying to install XP is a brand new notebook. Like skuzzy says, these may or may not have XP driver support. When I took it back to the store, the guy got on the phone to HQ, quoting my machine serial number. I could only hear half the conversation, but it sounded exactly as if he was being told that my system did NOT have XP driver support, but could only run Vista. It has a 200GB SATA drive, and like deselys says, I may have needed to say YES to SATA support at install time, but don't recall being asked. I did screw up one setting and ended up with only 127GB of my 200GB usable! - now sorted out using Acronis.
As for "someone wanting to sell me Vista", when I returned my copy of XP I received a store credit and then paid for Vista x64, which came to around $60 LESS than XP.