This is a bizarre problem - but if you get the stsadm.exe generic "command line error." whilst trying to install SharePoint web parts and your paths look fine, it may just be an encoding issue when you copy the commandline arguments between different apps. You do NOT need to have your wsp file in the same directory as stsadm.exe when installing parts to your site. You see, different apps interpret hyphens differently. If you copy a hyphen from a web site, it may just be a unicode representation of a hyphen and not a "real" hyphen. For more detail, see:
A simple solution is just to make sure you type all your stsadm.exe command parameters in manually and not copy and paste them into your DOS prompt.