I just installed Sun's virtualbox 2.x and had an extremely easy time installing XP Pro.
I did have to do a workaround as I wanted to have my vdi-files not setup in the default location (~/.virtualbox/) but on in the /usr/local/ area and the program did not support this. So I created a virtual XP setup to get the vdi-file and then deleted my virtual XP and created it again, this time just selecting my vdi-file now at my preferred location.
Now, I don't need to dual boot Linux and Windows, it's now straight Linux with Windows running inside of Linux. It is so much more convenient way to use Windows, as I do have a few Windows programs I still need/use.