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

Absolutely, even with Apple's provided Boot Camp solution. Consequently, EveryMac.com does not advise installing Windows on a "mission critical" Intel-based Mac. Of course, as a Mac site, EveryMac.com would not recommend that one use Windows.

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

In a "Requirements, Installation, and FAQ" document at the Apple Support Site, Apple provides the following warning:

Boot Camp Beta is preview software licensed for use on a trial basis for a limited time. Do not use Boot Camp Beta in a commercial operating environment or with important data. You should back up all of your data before installing this software and regularly back up data while using the software.

Apple does not provide technical phone support for using Boot Camp Beta, burning the Macintosh Drivers CD, or installing Windows XP. Support is available on Apple's website. Fee-based support agreements are not available for Boot Camp Beta.

On page 3 of the "Boot Camp Beta Setup and Installation Guide" Apple states more bluntly:

Apple Computer does not provide support for installing or running Boot Camp Beta or Microsoft Windows software.

Apple also has posted a "Boot Camp Assistant: Your Startup Disk Cannot Be Partitioned" document at the Apple Support Site. 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.

Remember that Boot Camp is beta software. Should you choose to install Windows XP on your Mac, even using Apple's software, back up everything and verify that the backup was successful before proceeding with your experiment.

With Parallels Desktop for Mac

To date (see top), EveryMac.com has not had readers report any major problems that were caused by installing the final version of Parallels Desktop for Mac. However, the software does install a kernal extension that modifies MacOS X, which is a bit more "dangerous" than the "average" MacOS X application.

Before installing Parallels Desktop for Mac, or any software, one should always back up everything and verify that the backup was successful before proceeding.

With VMWare Fusion

To date (see top), VMWare Fusion is only available in private beta, and EveryMac.com has not received any negative reports regarding the program. However, those testing the program undoubtedly know to only use beta software on systems that are not "mission critical".