I had exact same problem. Here is what steampowered has on it.

The this error indicates the friendsUI.dll file is corrupted or outdated.

The friendsUI.dll is extracted from the winui.gcf found in your
..\Steam\steamapps\ directory. To ensure you have the most recent working
friendsUI.dll, please delete or move the winui.gcf out of the steamapps
directory and restart Steam. The process will force Steam to download the
latest version of winui.gcf and re-extract your friendsUI.dll.

If deleting winui.gcf does not correct the issue, you must delete all
instances of friendsUI.dll to allow winui.gcf to re-extract the file: Run a
search for friendsui.dll in your Steam directory and delete the files, then
restart Steam.

This worked for me.

Jim

Hello, jrw2005!
You wrote  on Thu, 20 Sep 2007 00:38:05 -0700:

j> I attempted to login to my steam account last night, after an update I
j> kept getting the following:-

j> Platform Error: bad module 'Friends/friendsUI.dll' not loading.

j> has anyone had this error, and know of a fix.

j> Any help would be appreciated. Was trying to buy the Orange Box!!