Which Windows version to put on my MacBook?

I wanna put windows on my macbook for a few things that won't work on mac. Which version works best with the macbooks? Also should i use parallels or bootcamp?

I recommend any version of Windows XP. Windows Vista is unnecessary if you only need a few things. Earlier versions of Windows such as 98 or ME can work but they are not that stable. You do need to buy a new copy as you will have to register it. Make sure your using Windows XP with Service Pack 2 (SP2).
I use Windows XP Professional SP2 with both Bootcamp and Parallels. Parallels is great if you want both operating systems running at once. Bootcamp alone would save you the $79 for Parallels but you'll just have to reboot the Mac into Windows and vice-versa for Mac OS X… one OS at a time only. With Bootcamp your running fully native in Windows so the performance (even games) will run as good as any current PC. If your Mac came with 1 Gigabyte of RAM that would be OK for Bootcamp. If you go with Parallels I really recommend installing 2 GB of RAM because each OS needs a good 1 GB to run smoothly. By the way, Parallels will now run Windows from the Bootcamp partition if you have already installed Bootcamp so you can choose to do it both ways – boot up into Windows alone or run OS X and Windows at the same time with Parallels.

Personally if your Windows needs are light I'd go with Bootcamp. If you wish to run a Windows only program and you really need to have your iMovie session going at the same time, Parallels is WAY COOL!