Important: Backup your system with System Restore before trying this... Also, I assume you have some knowledge of working with regedit...
Open Regedit and navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion - Here, look for a sub key named RunServices, if it doesnt exist, create it... Within this, create an entry with name Bearshare and data command to execute (easy way is to set bearshare to startup and then look up the value in Run sub key)
This should work... I have done this with tally before...
Arun