You are unable to select or format hard drive partition when you try to install Vista

information   Information
When you try to install Windows Vista, you may experience one or more of the following symptoms:
  • The hard disk on which you want to install Windows Vista is not listed.
  • You cannot select a hard disk partition on which to install Windows Vista.
  • You cannot format a hard disk partition or partitions.
  • You cannot set the correct size for a hard disk partition.
Note   Note
What if you are unable to format a hard drive partition to install Vista?

Assuming that your hard drive works fine and you have a new vista installation dvd and its not working due to the above mentioned reason, then we will look at the DFFR (Debug F-disk Format Reinstall) method to delete the existing hard drive partitions and install vista on a blank hard drive.
warning   Warning
This will delete all the data on the hard drive and delete all the existing partitions including the hidden utility and diagnostics partition.

Here's How:

All you need is a resource disk or a boot Cd with the help of which you can boot the computer and enter a DOS windows or command prompt.
Boot the computer using the resource disk provided by your computer manufacturer. (Contact your system vendor tech support for more info).
You may get 2-3options: Select Exit to DOS or Command Propmt.
1) At the command prompt, type A: and press <Enter>.

2) From the A:\> prompt, type the following commands, and then press <Enter> after each command:
NOTE: Type the bolded text only (the debug prompt is a dash -). You may receive an error if you type anything other than the bold text. The non-bolded text represents the system response that will appear after typing each command and pressing the <Enter> key.

- A 100
xxxx:0100 INT 13
xxxx:0102 (Leave this line blank. Press <Enter> to continue.)
AX 0000: 0301
BX 0000: 0200
- F 200 L 200 0
CX 0000: 0001
DX 0000: 0080
- P
- Q

The screen returns to the A:\> prompt, and all data has been removed from the hard drive.