On the left is an Internet Explorer window. On the right is a Safari window. On the far right is my Macintosh dock. On the bottom is my Windows "Start" button. My Mac is simultaneously running both operating systems, and I can go back and forth from one to the other without even thinking about it.

This is Coherence, from Parallels. It's done with a bit of magic--a transparent window on the Windows desktop--and lots of clever programming from the folks at Parallels. You can copy on the Windows and paste into a Mac application, and vice versa. Or you can drag and drop from one environment to the other. The Windows applications can access all of your Mac files. The running Windows applications even appear in your Mac dock. It works really, really well.

So, what's the catch? There are several.

* Parallels is a memory hog. I have my virtual machine set to use 512 megabytes of real memory, but Parallels Desktop itself is using 2.1 gigabytes of virtual memory. If you want to switch back and forth between Windows and Mac applications, you really want 2 or 3 gigabytes of RAM. Wow.

* Parallels is cheap ($79), but you'll also need to buy a copy of Windows. That's why I'm running Windows 2000. Parallels can run Vista, of course, but it will cost ya.

* When you're running the Windows applications, you're, well, running Windows. So you'd better have antivirus, antispyware, and anti-everything-else installed. You don't have to give the Windows workstation access to your Mac files, but if you do, beware that you've given it the power to hurt you.

* Reboots and software updates. Remember, Microsoft frequently issues software updates, and they almost always require a system reboot. The good news is that Parallels boots fast. But if you don't leave it running all the time, you'll find that you need to install a bunch of updates every time you start it up. (Today I started it up, and the OS told me that I had to install Windows Direct 9.0 and reboot. When the Windows box came back up, it told me I had to install the security patches! And reboot! I'm sure glad that it boots fast.)

* Will this hurt Mac software developers? If you can trivially run the Windows version of an application on your Mac, what's the incentive for companies to develop Mac versions?

So, where does this leave me? Well, my Windows machine has been acting up lately. Perhaps I won't get a new one. Perhaps I'll just get a new iMac with 3 gigabytes of RAM. Perhaps I'll get a MacPro with 8 gigabytes of RAM.

Perhaps I'll never buy a PC that's not a Mac again.