Quoth Kyle Alons:
Set PrivilegesRequired=none and only write to non-admin-writable file/registry locations. For registering COM DLLs for a user, you probably won't be able to use the DLL's registering capabilities (unless you have the source and can modify the registration logic), but instead write the registration info under HKCU\Software\Classes instead of HKCR, or maybe use this tool:http://www.codeproject.com/KB/winsdk/regsvrex.aspx
As previously discussed on the newsgroups, registering COM objects per-user is a bad idea, since (a) it means they can no longer be serviced properly at the per-machine level [updating the per-machine copy to fix a security hole won't update the per-user copy, so your app will still have a security hole] and (b) most COM objects don't define a canonical per-user location, so if multiple apps want to register it per-user they'll do so in conflicting locations, leading to versioning hell.