Have you got just one RAID Controller in this machine ?
Is this a Home / Test Box or a Production Server ?
If it is a Production Box with a good raid controller and only 1 raid controller, making ONE partition would be good too as all disks are being managed by ONE Controller.
You can create a 80GB OS Partition, keeping in mind the OS/Windows Updates etc for the future, 50GB would do too.
If you can put your OS Disks onto a separate disk controller and your other 3 drives or 2 drives on a separate controller - that would be great.
But - in production we have servers running on ONE Raid controller with ONE partition for windows and vms, running 4 or 5 vms on it.
It all comes down to what sort of traffic you are anticipating on these VMs.
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Some tools :
You can use Vhdtool to create fixed size vhds instantly (fixed size VHDs are good and known for better performance then dynamic)
You can use Disk2VHD from Sysinternals / Microsoft to convert Physical Windows 2003 and onwards, servers to Virtual Machines, a very handy utility - does an online conversion.
Disk2Vhd : http://technet.microsoft.com/en-us/sysinternals/ee656415.aspxVHDto
ol :
http://code.msdn.microsoft.com/vhdtool
Hyper-V Networking : http://www.virtuatopia.com/index.php/Understanding_and_Configuring_Hyper-V_Virtual_Networks