Could installing Windows on the Mac cause problems with MacOS X?

With the final versions of software, proper installation procedure, and a supported version of Windows, Windows on the Mac should not cause problems with MacOS X.

Regardless, one always should backup their entire system before installing any software or before partitioning a hard drive as a preventative measure. Additionally, it is important to make regular backups to ensure important information is not lost due to software errors, data corruption, or hardware failure.

With "XOM"

This answer is provided primarily for "historical" purposes. Users who need to "dual boot" an Intel-based Mac would be better served by Apple's Boot Camp solution.

A MacWorld feature chronicled the process of installing Windows XP on an Intel-based Mac using "XOM". The full piece is well worth reading, but regarding the potential to cause problems, the author noted [emphasis added]:

If you're thinking about trying this on your own Intel-based Mac, you should know that Really Bad Things are possible. I'm not sure if permanent damage is possible -- I don't think that it is -- but you can easily get in a position where you might need to boot into single-user mode to recover. It's also possible to wind up in a situation where you have an unusable OS X, an unusable Windows XP, and the OS X installer refuses to see your drive as an available destination -- in fact, this happened to me twice. Thankfully, recovery is relatively straightforward, though you will lose all data on the machine.

With Boot Camp

For the beta versions, Apple noted that the company did "not provide support for installing or running Boot Camp Beta or Microsoft Windows software" and two readers reported that a problem occurred during partitioning that required them to erase the entire hard drive. A PC World article reported that after some users installed Windows XP on their Intel-based Macs using an early beta of Boot Camp, they were unable to boot back into MacOS X.

For the final version of Apple Boot Camp 2.0 provided with MacOS X 10.5 "Leopard" and Boot Camp 3.x provided with MacOS X 10.6 "Snow Leopard", properly installing a supported version of Windows on the Mac should not cause problems with MacOS X. Nevertheless, some have encountered difficulties, and regardless, one should back up everything and verify that the backup was successful before partitioning, installing, or configuring Boot Camp.

With Parallels Desktop for Mac

Installing a final version of Parallels Desktop for Mac should not cause problems with MacOS X, but nevertheless, before installing Parallels Desktop for Mac, or any software, one should always back up everything and verify that the backup was successful before proceeding.

You may also wish to review Parallels Desktop for Mac reader feedback from the always excellent MacInTouch.

With VMWare Fusion

Installing a final version of VMWare Fusion should not cause problems with MacOS X, but nevertheless, before installing VMWare Fusion, or any software, one should always back up everything and verify that the backup was successful before proceeding.

You may also wish to review VMWare Fusion reader feedback from the always excellent MacInTouch.