If u uses Window Vista Enterprise or Ultimate and want to use the BitLocker Encryption, but your computer does not support the TPM module. In that case, u may use a USB stick or a memory card as the key medium, then u’ll have to force Vista to accept something other than the TPM module.
Steps are:

1. Goto Run and type ‘gpedit.msc’ and press Enter.

2. Now goto ‘Computer Configuration > Administartive Templates > Window Components > BitLocker Drive Encryption’

3. Now open up configuration windows by clicking on ‘Control Panel: Advanced startup Options’

4. In the ‘Settings’ tab select ‘Active ‘ and ‘Allow BitLocker without a compatible TPM’.

5. After clicking Accept , the BitLocker wizard (in Control Panel) shows the newly added option.

After this , do rest of the setting according to the instructions given in the control panel. Before encrypting any hard disk, make sure u have take a backup copy of ur disk.