You have two good options for running Windows on your Intel iMac: Parallels Desktop and VMware Fusion. I use and recommend Parallels although lots of people prefer VMware. If you aren’t the adventurous type, just get Parallels. If you are using the same thing I am, there’s a much better chance that I’ll be able to help you.
If you plan to run Windows XP or Vista, make sure you have at least 3 and preferably 4GB of RAM. Any less and you’ll have severe performance problems when your system runs out of memory and starts paging. It won’t be an enjoyable experience.
If you have an older Intel iMac that only supports 2GB of RAM, then forget about either and use Boot Camp which comes with Mac OS X 10.5. Then you can partition your drive and use part of your disk for Windows. The disadvantage is that you’ll have to reboot to use any Windows apps. But that is far better than trying to run Windows without enough RAM.