While others have recommended WINE (and, no doubt, have had tremendous success with it), I have to recommend win4lin. (
http://www.netraverse.com/products/win4lin50/)
It's not free ($90), but it's paid for itself hundredfold. I frequently had applications crash when trying to use them under WINE (Flash MX and Illustrator are two biggies for me), or they suffered the crippling of important features. Win4lin lets you run a full shell with the OS in it and the app of your choice under that session.
And you still get the benefit of being able to ALT-TAB to a good IDE like Quanta to write your PHP scripts, while working across the LAN with Samba under Linux (which is better by far than trying to get Window$ to recognize and use your Samba shares...)
I know not free is not popular, but sometimes you just have to get the work done...; It's been two years -- maybe I'd have a different experience with WINE if I tried it today.
My $0.02.