I didn't know your password.
In any case, I've done 3 installs without an issue (MBP CD twice and Mini CD). You SHOULD always back up because you never know. Besides, what would happen if you hard drive took a dump - you would be up sh*t creek, that's for sure.
If you can read and comprehend what is in the manual, you will be fine. Of course, by read I mean - you:
1) use an officially supported operating system and valid install disc
2) don't bother trying if your HDD is already partitioned
3) select the correct partition when installing
4) don't try to resize partitions after the fact - don't make your decision on partition size without proper planning
The only negative is the fact that Windows is on my mac.