You need XLiveLess to load other savegames. GTAIV locks other peoples' savegames even if it's yours and you reformatted your PC. Also there's no such thing as 64bit savegames. If you want to go on LIVE while playing that savegame, play the first mission in IV and save the game, then go to the %APPDATA%/Local/Rockstar Games/GTAIV/Savegames folder, open that user_(numbers) folder and replace that savegame with the one you had before.