Should be possible. Nexus' installation (which runs as administrator) probably created the registry keys at...
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mithis\Nexus - The Jupiter IncidentBut the game running without administrator privileges looks for them here:
HKEY_CURRENT_USER\Software\Classes\VirtualStore\MACHINE\SOFTWARE\Wow6432Node\Mithis\Nexus - The Jupiter IncidentYou can run "regedit" (hit Windows-key and [R], type "regedit"), navigate to the first path and create these same values at the second path, where they belong. Or, a faster way:
- start up "regedit"
- navigate to the first path above, HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Mithis\Nexus - The Jupiter Incident
- right-click the "Nexus - The Jupiter Incident" registry key and choose "Export"
- save it somewhere as "nexus.reg" (or whatever)
- open the file in a text editor and replace the path starting with "HKEY_LOCAL_MACHINE..." with the correct one (the second from above): HKEY_CURRENT_USER\Software\Classes\VirtualStore\MACHINE\SOFTWARE\Wow6432Node\Mithis\Nexus - The Jupiter Incident
- save the changes and run the .reg file by double clicking
This should add an identical copy of the correct registry values into the correct location and the game should start up fine now without the need to run as administrator.
I know it's really inconvenient, but Vista and Windows 7 just handle registry access a bit different than previous Windows versions and Nexus was developed years before that. And with the developer shutdown there won't be any patch to ever fix that issue.