You can get information about errors and how to use the API at this site...
http://weblogs.asp.net/junfeng/archive/2004/09/29/235620.aspx
What are the arguments you are passing to the call to InstallAssembly? I use it all the time and it works for me. Make sure you are using a valid TraceReference.
Here's an enum I use that maps COM error codes to a .net type
/// <summary>
/// A map from HRESULTS to a .net type. Reference file
/// CorError.h for a complete list.
/// </summary>
public enum GACErrorCodes : uint
{
/// <summary>
/// This error indicates that during a bind, the assembly reference did
/// not match the definition of the assembly found. For strongly-named
/// assemblies, if any of the tuple (name, version, public key token,
/// culture) do not match when binding to an assembly using
/// Assembly.Load (or implicitly through type resolution), you will get this error.
/// For simply-named assemblies mismatches in the tuple (name, culture)
/// will cause a ref-def mismatch.
/// </summary>
FUSION_E_REF_DEF_MISMATCH = 0x80131040,
/// <summary>
/// Private (or "simply named") assemblies are restricted to live only under the
/// appbase directory for a given domain. This restriction is necessary to prevent
/// unintended sharing of the assembly. Since the identity of simply-named is
/// not unique, it is possible name collisions can occur, and therefore,
/// it is not safe to share the assembly across apps/domains. If a
/// simply-named assembly is ever loaded via Assembly.Load that does not appear
/// underneath the appbase, this error will be returned.
/// </summary>
FUSION_E_INVALID_PRIVATE_ASM_LOCATION = 0x80131041,
/// <summary>
/// When installing an assembly in the GAC, if a module (part of a
/// multi-file assembly) listed in the manifest is not found during
/// installation time, the install will fail with this error code. Modules
/// should be present alongside the manifest file when installing them into the GAC.
/// </summary>
FUSION_E_ASM_MODULE_MISSING = 0x80131042,
/// <summary>
/// When installing multi-file assemblies into the GAC, the hash of each module is
/// checked against the hash of that file stored in the manifest. If the
/// hash of one of the files in the multi-file assembly does not match what is recorded
/// in the manifest, FUSION_E_UNEXPECTED_MODULE_FOUND will be returned.
///
/// The name of the error, and the text description of it, are somewhat confusing.
/// The reason this error code is described this way is that the internally,
/// Fusion/CLR implements installation of assemblies in the GAC, by installing
/// multiple "streams" that are individually committed.
///
/// Each stream has its hash computed, and all the hashes found
/// are compared against the hashes in the manifest, at the end of the installation.
/// Hence, a file hash mismatch appears as if an "unexpected" module was found.
/// </summary>
FUSION_E_UNEXPECTED_MODULE_FOUND = 0x80131043,
/// <summary>
/// This error is returned when a simply-named assembly is encountered,
/// where a strongly-named one was expected. This error code could be returned,
/// for example, if you try to install a simply-named assembly into the GAC.
/// To strongly-name your assembly, use sn.exe.
/// </summary>
FUSION_E_PRIVATE_ASM_DISALLOWED = 0x80131044,
/// <summary>
/// Both at bind time, and install-time, strong name signatures of assemblies are
/// validated. Every strongly-named assembly has a hash of the bits
/// recorded in the manifest (signed with the private key). If the hash of
/// the assembly found does not match that recorded in the manifest, signature
/// validation fails.
/// </summary>
FUSION_E_SIGNATURE_CHECK_FAILED = 0x80131045,
/// <summary>
/// There are a variety of conditions which can trigger this error, most of
/// which are related to either poorly-formed assembly "display names"
/// (textual identities), or path-too-long conditions when translating an
/// assembly identity into a corresponding location on the file system.
/// </summary>
FUSION_E_INVALID_NAME = 0x80131047,
/// <summary>
/// This error indicates that AppDomainSetup.DisallowCodeDownload has been set
/// on the domain. Setting his flag prevents any http:// download of
/// assemblies (or configuration files). ASP.NET sets this flag by default
/// because the internal usage of wininet/urlmon to download bits over
/// http:// is not supported under service processes.
/// </summary>
FUSION_E_CODE_DOWNLOAD_DISABLED = 0x80131048,
/// <summary>
/// When the CLR is installed as part of the operating system
/// (such as in Windows Server 2003), uninstalling CLR or frameworks assemblies
/// is disallowed.
/// </summary>
FUSION_E_UNINSTALL_DISALLOWED = 0x80131049,
}
