Hmmm… A couple of things to try.
* Check if any of your project dlls are on the biztalk installation folder (usually c:\Program Files\Microsoft BizTalk Server 2004).
* Un-gac all the versions and references of you solution dlls by running gacutil with the /uf parameter
* If your solutions involve IIS also restart IIS
* Terminate all the suspended and zombie and other instances you might have in BizTalk (this script might be useful http://msdn.microsoft.com/en-us/library/bb203857.aspx)