My brother in law bought a Dell's Inspiron 1520, he asked me to install Windows XP on it. It has Vista Basic pre-installed.
I thought it will be a straight forward process, but all my install attempts ended up in the dead blue screen and I had to resort to re-boot the laptop. After the 2nd blue screen, I decided that it's probably too much work to fiddle around, I am pretty sure it can be done. I have done quick googling, while I didn't find any links showing how to replace Vista with XP, I found some good links detailing how to have Vista and XP dual boot, well I don't think my brother in law is that desperate for XP that he wants to sacrifice disk space to have both OSes running.
Vista does feel a little different to XP though, glad I don't have to personally use it yet.