Linux.
It can be tricky to install on laptops (but so can Windows: most Windows users don't know about installation problems because they buy it pre-installed on their PCs)

I don't want to get into an OS war here...
A Mac almost certainly doesn't have fragmentation problems either.
Actually I didn't think NTFS (the file system you should be using with Windows XP) was too bad for fragmentation either.

Anahata