Linux - Linspire or Xandros. I use both - both are graphical, simple to follow after Windows - install straightforwardly - include browser, email, Office Suite, etc.
Linspire is a commercial version - Xandros is also a commercial version, but also has a free download version. Linspire has more ready-to-install and run software. Lspire is one licence only for all your systems (in a household).
Both will dual boot with WinXP.
I recommend you check them both out.