By default all hard drives in windows vista always show but you can hide some or all your hard drives so that nobody can open it or if you are parents and avoid remove or delete system file by your children, it is best for you that hide this hard drive (Generally C drive )
With this tip you can only hide drive but  you can access your data and open drive with dos or typing drive name directly in The Address bar like c:\ and press enter.

Ok you can do this with two methods

First method: Through the Local Group Policy Editor (More Easy)
Follow these steps:
1. Click on the Start Menu.
2. In the Search bar, type gpedit.msc and press Enter
Or
Press Window Logo + R type gpedit.msc and press Enter.

3. Click Continue for the UAC prompt. (If you have active the UAC service)

4. (In the left pane) click on User Configuration, Administrative Templates, Windows Components, and Windows Explorer.
5. (In the right pane) Double Click on Hide these specified drives in My Computer.
6. Click on Enabled Radio button and choose your drive which you want to hide then click on apply and ok.

7. If you have already hide drive one or all and this time you want to unhide choose
Don’t restrict all drives
Or
Click on not configured Radio button then apply and ok
And After this log off or restart your system.

You can also use this in Window xp

Note: First method is good for you if you want to hide C or D single drive or hide your all drive but if you want to hide other hard drive like E, F or G or you want to hide only F drive or G drive you don’t this Through the Local Group Policy Editor but you can do this with change registry setting.

Second method: Through the Registry (Easy)
Use this method when first method is not work for you:
1. Click on the Start Menu.
2. In the Search bar, type regedit and press Enter
Or
Press Window Logo + R type regedit and press Enter.
3. Click Continue for the UAC prompt. (If you have active the UAC service)
4. In regedit go to:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
5. In the right pane of Explorer, right click on an empty area and click on New and DWORD (32-bit) Value and type NoDrives (without any space) and press Enter.

6.  Right click on NoDrives and click Modify.

7. Now you can use these value which drive you want to hide.

please choose value according to base if you choose Hexadecimal use hexadecimal value but if you want to choose Decimal value use Decimal value.

8. Close Regedit And Logoff and logon again, or restart the computer to apply changes.

To unhide drives –
In the right pane of Explorer, right click on NoDrives and click Delete and yes to confirm delete.
Close Regedit And Logoff and logon, or restart the computer to apply changes.