With Parallels the Mac runs both Windows and MacOS applications.  Each in separate windows, but you really can't tell which is which.  You can be running 5 different Windows applications and 5 different Mac applications, each in 10 differnt windows.  You can cut and paste between them and the files are all stored in the same folders.  You can put Windows application icons on the startup Dock just like Mac applications.  Windows uses the same printers.  It's nearly seamless.

I think Parallels costs about $75.