Hi zahidmughal and welcome to Seven Forums. If you mean assign a drive letter to a vhd then this is the way Windows 7 is meant to behave. I don't think there is a way to change this.
Computer type PC/Desktop System Manufacturer/Model Number Hewlett-Packard/G62-107SA Notebook OS Microsoft Windows 7 Home Premium 64-bit 7601 Multiprocessor Free Service Pack 1 CPU Intel(R) Core(TM) i3 CPU M 330 @ 2.13GHz Motherboard Hewlett-Packard 1425 Memory 8 GB DDR3 Graphics Card Intel(R) HD Graphics Sound Card Realtek High Definition Audio Monitor(s) Displays Builtin Screen Resolution 1366 x 768 x 32 bits (4294967296 colors) @ 60 Hz
Mouse Microsoft Bluetooth Notebook Mouse 5000 Hard Drives 250 GB SATA Hard Disk Drive 7200 rpm
2TB Seagate GoFlex USB 2 Drive
1TB Iomega Prestige USB 2 Drive
1.5TB Iomega Prestige USB 2 Drive (Samsung)
1TB Iomega NAS. Internet Speed 60 Mbs download 3 Mbs upload Antivirus Norton 360 Browser Chrome
Computer type PC/Desktop System Manufacturer/Model Number Hewlett-Packard/G62-107SA Notebook OS Microsoft Windows 7 Home Premium 64-bit 7601 Multiprocessor Free Service Pack 1 CPU Intel(R) Core(TM) i3 CPU M 330 @ 2.13GHz Motherboard Hewlett-Packard 1425 Memory 8 GB DDR3 Graphics Card Intel(R) HD Graphics Sound Card Realtek High Definition Audio Monitor(s) Displays Builtin Screen Resolution 1366 x 768 x 32 bits (4294967296 colors) @ 60 Hz
Mouse Microsoft Bluetooth Notebook Mouse 5000 Hard Drives 250 GB SATA Hard Disk Drive 7200 rpm
2TB Seagate GoFlex USB 2 Drive
1TB Iomega Prestige USB 2 Drive
1.5TB Iomega Prestige USB 2 Drive (Samsung)
1TB Iomega NAS. Internet Speed 60 Mbs download 3 Mbs upload Antivirus Norton 360 Browser Chrome
Computer type Laptop System Manufacturer/Model Number ACER ASPIRE 5742G OS Microsoft Windows 7 Home Premium 64-bits 7601 Multiprocessor Free Service Pack 1 CPU Intel(R) Core(TM) i3 CPU M 370 @ 2.40GHz Motherboard Acer Aspire 5742G Memory 4,00 GB Graphics Card ATI Mobility Radeon HD 5400 Series Sound Card (1) AMD High Definition Audio Device (2) Realtek High Defi Screen Resolution 1366 x 768 x 32 bits (4294967296 colors) @ 60 Hz