Thank you for your post.
I'm not sure if it is going to work or not. We didn't certified FileMaker Server 10 with R2, but users seem to be working with it fine.
I would suggest uninstalling FileMaker Server 8 prior to updating to R2. After uninstalling, update to Windows Server 2003 R2, and then install FileMaker Server 8. If this doesn't work, then you'll have to uninstall FileMaker Server 8 and revert back to R1.
Thanks For your usefull reply and support,let me try out as per your provided solution and direction,if i'll get any difficulties than surely contact you.