Parallels Workstation runs Windows apps seamlessly within OSX on the new Apple Intel Macs such as the MacBook Pro 17" notebook computer (shown here).  It runs them at nearly 100% speed, and in some cases in faster than on a PC.  As you can see, the Parallels Workstation running Windows XP can simply appear as a window within the OSX environment, so you can switch back and forth between windows.  Within the Workstation window you can also run multiple Windows programs.  In the example below, I have about 10 Windows programs open (including Ensign, mIRC, Trader Workstation, Yahoo Messenger, Snagit, Microsoft FrontPage, and browsers).  I also have several programs running on my Mac at the same time, including Photoshop, Mail, Calendar, Microsoft Word and Excel. All are running fast and smoothly.

This shows Parallels Workstation running at 1440 x 852 resolution as a window within OSX on my MacBookPro 17" with  1680 x 1050 resolution.  Note all of the programs that are open.  I have dedicated 900mb of RAM to the XP workstation, and 1100mb to OSX.  Scroll down to see all programs that are open.

This shows an Expose' view of the entire 1680 x 1050 Desktop, which shows every single application and window that is open (Expose is a wonderful feature.  I only wish I had one on my real desktop!)  In it you can see that the Parallels Workstation virtual machine running Windows XP appears as just one of many windows.