Virtual PC lets you run MS Windows on your Mac. But...
...it's useless and irrelevant if you have an Intel Mac;
...(although I've never used it) it performs badly on a PPC anyway (from what I've heard);
Let me clarify this:
I own a PPC G5 Quad, with 4 GB RAM
I have Virtual PC 7 installed, running Windows 2000 Pro (SP 4).
Windows has 512 MB of memory assigned (max), and max. settings for "video memory".
The performance is dreadful. Yes, you can use Word / Excel and browse the internet, or maybe even do your income tax returns, because these apps don't need a "quickish" CPU. Try installing QuickTime, and playing a simple 640 x 480 MPEG4 encoded movie.... forget it. Maybe 10 fps tops.
Becasue EVERY action must be "translated" bewteen x86 and PPC, it all takes ages for evey action in Windows to complete. Even my Quad 2.5 GHz G5 will struggle to outperform a P3 running at a mere 400 MHz.... Moving a window around with the contents visible shows so much lag, it looks like running Mac OS X 10.0.0 on an iMac G3 233 MHz with 2 MB VRAM and 32 MB RAM.
On top of that, the grfx is also emulated. The Video-RAM is borrowed from the normal RAM, which is slower than normal VRAM and probably accessed at a very, very slow speed aswell. Example above illustrates that to some extent too.
So, it simply is a tool to let you run some non-CPU intense and non-grfx demanding Windows-only applications on your Mac.
I used to use it for my income tax returns, but fortunately we have a Mac version over here now.
But, I still install it for the fun of it.