Very easy. Unless you want to keep all your personal files and programs:
The system for upgrading is complicated, but Vista owners can upgrade to the exactly comparable edition of Windows 7 while keeping all files, settings and programs in place.
Unfortunately, XP owners, the biggest body of Windows users, won’t be able to do that. They’ll have to wipe out their hard disks after backing up their files elsewhere, then install Windows 7, then restore their personal files, then re-install all their programs from the original CDs or downloaded installer files. Then, they have to install all the patches and upgrades to those programs from over the years.