I would strongly recommend using the Wix tool Heat.exe to harvest all the data needed to register the com component and then reference the fragment in your .wxs file like this:
<ComponentGroupRef Id="FooBar.dll" />
Or include it in your .wxs file like this:
<?include FooBar.dll.wxi?>
This method gives you full control over what happens during the registration/Unregistration of the Com component.
You can however still use Regsvr32 in a Wix project. But it relies on the correct implementation of the RegisterServer/UnregisterServer functions in the COM component
<CustomAction Id="RegisterFooBar"
Directory="INSTALLDIR"
ExeCommand='regsvr32.exe /s "[INSTALLDIR]FooBar.dll"'>
</CustomAction>
<CustomAction Id="UnregisterFooBar"
Directory="INSTALLDIR"
ExeCommand='regsvr32.exe /s /u "[INSTALLDIR]FooBar.dll"'>
</CustomAction>
Then add your action to the install sequence.
<InstallExecuteSequence>
<Custom Action="RegisterFooBar" After="InstallFinalize">NOT Installed</Custom>
<Custom Action="UnregisterFooBar" After="InstallFinalize">REMOVE="ALL"</Custom>
</InstallExecuteSequence>