I'd just heard conflicting statements and came here to see who was right.
In theory, if your hard drive is partitioned with the Windows operating system on C: and everything else on D:, you supposed to be able to reinstall Windows without losing anything. In theory. There are many stories where it did work, and many stories where it did not. That's why you're hearing conflicting statements.
roaminggnome is a windows user, not a pretentious Apple user who told you to "buy a Mac". RG just cuts to the chase. You need a full backup.
Like Walter pointed out, you'll get better help if you provide more details. The first being, what OS are you using now, and what OS do you intend to go to? Just as I asked in my first reply.
Walter has a success story becuase he upgraded his OS and I suspect his drive was partitioned correctly before he did the upgrade. For all we know, you're planning on going from a non-partitioned Windows XP installation to Linux, which would result in disaster.