The registry entry that holds the pointer to the signature file is:

HKEY_CURRENT_USERSoftwareMicrosoftOffice10.0CommonMailSettingsNewSignature (new messages)
HKEY_CURRENT_USERSoftwareMicrosoftOffice10.0CommonMailSettingsReplySignature (replies)

Of course you'd need to modify the path to whatever version of Office is on your PCs. The signatures themselves are stored in:

"C:Documents and SettingsProfileNameApplication DataMicrosoftSignatures"

Each signature creates three versions of itself, text, RTF, and HTML to handle the three different message formats Outlook supports. You could add the signature files as you install Outlook or add them later via a script (as I mentioned above), to include setting the default signature. You could even add the signature copy to a login script, thereby giving yourself a mechanism to update the signature as changes are made. The only thing I don't see a way to control is keeping each user from changing their signature to something else. For that you might want to have a look at system policies and see if there's some way to lock the signature down that way. Policies might also be another way to copy the signatures to each computer.