The Stock Pack and Expansion Pack both allow you to race HS live online with the rest of the racing community. To learn more about the Stock Pack/Expansion Pack, keep on reading 🙂 .
Stock Pack 3
There is a single installation pack needed to update NFS High Stakes directly from your CD-ROM to be able to properly race online with the Collision Off option. The NFS High Stakes STOCK PACK Installer (3MB) includes patches to ensure a smooth online experience, and also includes all the key patches to get the game working on Windows XP and newer hardware. It is strongly recommended that you also read the NFS HS Online Racing Guide for step-by-step installation instructions and lots of troubleshooting tips.
Expansion Pack 3
The Need For Speed High Stakes expansion pack is a package that contains everything that the Stock Pack has, plus an amazing assortment of hand-picked cars and tracks from among the many user-modified/created cars and tracks for High Stakes, available from many download sites. IP Lounge and this database have been customized to integrate and include these tracks and cars for official Online racing and scoring within this system. The design of High Stakes is such that to accomplish this, tracks must replace original tracks in the game in order to be available. These extra tracks and cars are not meant to REPLACE anything in the original game. The sole intent is to expand the game to include an often overlooked aspect of the history of the Need for Speed series of games, and to increase the fun factor and diversity a bit. In addition to these add-ons, the Expansion Pack includes the IP Lounge, the NFS HS Mixer (customized for this Pack and IP Lounge), the No Collision Patch, the No-CD patch (requires copying some files from the CD to the game folder) and some custom game menus. This EP is the product of thousands of hours of work by High Stakes racers for the love of the game, it is totally free, and you can DOWNLOAD IT HERE (80MB).
Enjoy,
-FerrariMan
FM HS Mod Site
NFSMods
Blazn Motorsports
HS Scoring
IPLounge Forums
NFSAddons
FM's HS Site 
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