Suppose you want to create 20gigs VHD in drive C with name win7.vhd and assign it into drive H
Open command prompt with administrator right
Type diskpart
CREATE VDISK FILE="c:\win7.vhd" MAXIMUM=20480
SELECT VDISK FILE="c:\win7.vhd"
ATTACH VDISK
CREATE PARTITION PRIMARY
ASSIGN LETTER=H
FORMAT QUICK LABEL=Windows7
EXIT
