i was in the same position as you but i found out a way of doing it.
1. Copy the files, SGTA4xx, from the folder "C:\Users\XXXXX\AppData\Local\Rockstar Games\GTA IV\savegames\user_xxxxxxxxxxxxxx\" (for vista) or "C:\Documents and Setting\XXXX\Application Data\Local\Rockstar Games\GTA IV\savegames\user_xxxxxxxxxxxxxxxx\ (XP) on your old installation to a safe place.
*NOTE, make sure u have 'show hidden files and folders' checked in Tools>Folder options>View
2. Go to your new installation and create a new Games for Windows live offline account.
3. Download XLive Wrapper from:http://blog.gib.me/wp-content/uploads/2008...rapper_1207.zip
(this is for the new version of GTA4 so install the patch first)
4. Read the instructions, then install Xlive Wrapper.
5. Copy the old saved game files and put them under the new installation "C:\Users\XXXXX\AppData\Local\Rockstar Games\GTA IV\savegames\user_xxxxxxxxxxxxxx\" folder.
6. Run the game, cross your fingues and try to load the saved games. (should work) if it didnt, try re-installing Xlive Wrapper.
Hope it helped.