A third option is CrossOver. It's currently a Linux-only app, although the company has expressed interest in making an Intel Mac version too. Essentially, it's a Windows emulator, and will therefore allow you to run Windows apps.
The difference between CrossOver and Virtual PC is what it emulates. VPC emulates a PC, and you install Windows on it. CrossOver emulates Windows itself, so you don't have to buy a Windows licence. Furthermore, since they have all the pseudo-Windows source code, they can customise it and deliver a more Mac-like experience
Since CrossOver is an incomplete emulation, not all software will run. The Linux version's compatibility list is
here. Most of the apps in this list have been tested and submitted by users. Officially supported apps (apps that they will provide technical support for if you can't get them running) are Word, Excel, PowerPoint, Outlook, IE, Access, Project, Photoshop, Visio, Lotus Notes, Quicken, iTunes, FrameMaker, QuickTime, Shockwave and Windows Media Player. Remember that this list is for the Linux version - a lot of these apps are available in native Mac versions.