You can use a WinPE USB stick to boot but it must include the winpe-vmi package:
To create an image that can decrypt your first computer you'll need another computer with WAIK installed.
1.1 How to create a WinPE image that suits your needs: http://technet.microsoft.com/en-us/library/dd744533(v=ws.10).aspx
1.2 You should include the winpe-wmi package when adding packages.
Dism /image:C:\winpe_x86\mount /Add-Package /PackagePath:"C:\Program Files\<version>\Tools\PETools\<architecture>\WinPE_FPs\winpe-wmi.cab"
Dism /image:C:\winpe_x86\mount /Add-Package /PackagePath:"C:\Program Files\<version>\Tools\PETools\<architecture>\WinPE_FPs\en-us\winpe-wmi_en-us.cab"
You'll need both: the language neutral and the language specific (en-us) package. version can be Windows OPK or Windows AIK (WAIK).
1.3 Boot your first computer (the one where the password is lost) with WinPE and execute the following command:
cscript %systemroot%\system32\manage-bde.wsf –unlock Volume : -rk PathToExternalKeyFile: \ External Key File Name
If the Recovery Key is damaged or missing, you can still unlock the volume. To do this, type the following command at the command prompt:
cscript %systemroot%\system32\manage-bde.wsf –unlock Volume : -rp Numeric Recovery Password
source:
http://social.technet.microsoft.com/Forums/en-US/w7itprosecurity/thread/984ca855-43ae-487c-ae6e-edd955b5d956/