Okay, so you're able to launch singleplayer but not multiplayer. Assuming you mean IV:MP, do you have your client.log for us? That would be able to tell us more.
Wont open but Crash Info
-------------------------------------------------------------------
Version identifier: IV:MP 0.1 T3 (Apr 15 2011, 17:52:37)
Exception address: 0x00479346
Game base: 0xFFED0000
Game state: 0x00000000
Exception code: 0xC0000005 (EXCEPTION_ACCESS_VIOLATION)
Edi: 0x00000002 Esi: 0x00000001 Ebx: 0x00000000
Edx: 0x77107074 Ecx: 0x33971C47 Eax: 0x00000001
Ebp: 0x03F44040 Eip: 0x00479346 Esp: 0x01C4EE48
Local player id: -1
Minidump written?: Yes
Module: ntdll.dll Base address: 0x770C0000
Module: kernel32.dll Base address: 0x76FE0000
Module: KERNELBASE.dll Base address: 0x75370000
Module: USER32.dll Base address: 0x75570000
Module: GDI32.dll Base address: 0x76CB0000
Module: LPK.dll Base address: 0x77210000
Module: USP10.dll Base address: 0x76F40000
Module: msvcrt.dll Base address: 0x76B20000
Module: PSAPI.DLL Base address: 0x76BD0000
Module: CRYPT32.dll Base address: 0x751E0000
Module: MSASN1.dll Base address: 0x750D0000
Module: WININET.dll Base address: 0x768E0000
Module: api-ms-win-downlevel-user32-l1-1-0.dll Base address: 0x75330000
Module: api-ms-win-downlevel-shlwapi-l1-1-0.dll Base address: 0x750E0000
Module: shlwapi.DLL Base address: 0x75A50000
Module: api-ms-win-downlevel-version-l1-1-0.dll Base address: 0x753E0000
Module: version.DLL Base address: 0x74620000
Module: api-ms-win-downlevel-normaliz-l1-1-0.dll Base address: 0x751D0000
Module: normaliz.DLL Base address: 0x77220000
Module: RPCRT4.dll Base address: 0x76BE0000
Module: iertutil.dll Base address: 0x75640000
Module: api-ms-win-downlevel-advapi32-l1-1-0.dll Base address: 0x75320000
Module: advapi32.DLL Base address: 0x76EA0000
Module: sechost.dll Base address: 0x753F0000
Module: USERENV.dll Base address: 0x750F0000
Module: profapi.dll Base address: 0x750C0000
Module: binkw32.dll Base address: 0x18000000
Module: WINMM.dll Base address: 0x70FE0000
Module: WINTRUST.dll Base address: 0x751A0000
Module: xlive.dll Base address: 0x10000000
Module: d3d9.dll Base address: 0x6ECB0000
Module: d3d8thk.dll Base address: 0x6EE80000
Module: dwmapi.dll Base address: 0x738A0000
Module: DINPUT8.dll Base address: 0x5C370000
Module: DSOUND.dll Base address: 0x6E2F0000
Module: ole32.dll Base address: 0x75410000
Module: POWRPROF.dll Base address: 0x744E0000
Module: SETUPAPI.dll Base address: 0x76D00000
Module: CFGMGR32.dll Base address: 0x75340000
Module: OLEAUT32.dll Base address: 0x77270000
Module: DEVOBJ.dll Base address: 0x753C0000
Module: WS2_32.dll Base address: 0x77230000
Module: NSI.dll Base address: 0x77300000
Module: WMVCore.DLL Base address: 0x71E20000
Module: WMASF.DLL Base address: 0x71DE0000
Module: gdiplus.dll Base address: 0x739D0000
Module: SHELL32.dll Base address: 0x75B00000
Module: ATL80.DLL Base address: 0x630B0000
Module: apphelp.dll Base address: 0x74F90000
Module: AcLayers.dll Base address: 0x67280000
Module: SspiCli.dll Base address: 0x74F60000
Module: WINSPOOL.DRV Base address: 0x745C0000
Module: MPR.dll Base address: 0x701F0000
Module: AcGenral.DLL Base address: 0x57BC0000
Module: UxTheme.dll Base address: 0x73B60000
Module: samcli.dll Base address: 0x735F0000
Module: MSACM32.dll Base address: 0x6C7D0000
Module: sfc.dll Base address: 0x734B0000
Module: sfc_os.DLL Base address: 0x6AEF0000
Module: urlmon.dll Base address: 0x75880000
Module: api-ms-win-downlevel-ole32-l1-1-0.dll Base address: 0x75310000
Module: AcXtrnal.DLL Base address: 0x57960000
Module: SHUNIMPL.DLL Base address: 0x6E4D0000
Module: SortServer2003Compat.dll Base address: 0x6AE20000
Module: IMM32.DLL Base address: 0x76C90000
Module: MSCTF.dll Base address: 0x76750000
Module: Client.dll Base address: 0x572C0000
Module: d3dx9_38.dll Base address: 0x56EF0000
Module: libcurl.dll Base address: 0x002A0000
Module: WLDAP32.dll Base address: 0x75AB0000
Module: MSVCR90.dll Base address: 0x70320000
Module: dbghelp.dll Base address: 0x6F550000