What's the least expensive way to get Windows on a Mac?

I recently purchased an imac and want to know where I can get the least expensive version of Windows. It doesn't have to be 7, Windows XP will do just fine.

Is OEM for system builders O.K. to use on a mac?. I just don't want to pay $200 for an operating system to run a couple of programs.

Yes, an OEM for system builders will work.

XP must have SP2.

I second that. I have an OEM disc of Windows XP Professional SP2 which I bought for around $130 back in Feb 2009.

It worked perfect for me.

If you're looking for Windows XP Home, you can currently get the disc for $99 at Fry's Electronics (Fry's Home Electronics | Computer Parts & Accessories, Software, Games, TVs, Cameras - Frys.com)

Thanks guys. I just picked up a copy of Windows 7 OEM 32 bit at Microcenter for $99.00 and Parallels 5.0 at Best Buys for $69.00

I originally wasn't going to get 7 but there wasn't any price difference from Xp to vista to 7. I was glad I asked the Apple specialist at Microcenter, whether to get the 32 or 64 bit version. He said that there were some problems with the 64 bit with regards to networking.

I'm just glad that I was able to get what I needed to run Windows on my Mac for a reasonable price. Now I can run Microsoft Publisher, which I have close to 100 documents for my business and my daughters school projects. I will also be running Quickbooks and and I have a, business specific, program that I want to access my files on.