HKCU\Identities\{The OE Identity's GUID}\Software\Microsoft\Outlook Express\5.0
 \signatures\Number of the signature

create a new DWORD value named "end" and associate the content "1"
(one) to it. That will lead to the signature beeing placed under your
posting but *in front of* messages content you're quoting on the bottom.

If you prefer to reply at the bottom you'll need to manipulate another
registry key:

HKCU\Identities\{The OE Identity's GUID}\Software\Microsoft\Outlook Express\5.0

create a new DWORD value named "Reply At End" and associate the
content "1" (one). That will leed to bottom postings with the
signature beeing placed at the very end.

In case you want a combination of both (top posting but signature
at the very end of a message) you're lost with OE.

Make sure you backup your registry *before* humbleing through it!