Lemmeh break it down for yaz:
1) Insert your HaloPC CD into your CD/DVD drive
2) Explore the cd using whatever means you feel comfortable with (IE, Explorer, etc). From here on I'll refer to the CD drive as drive G:
3) Go into the folder labeled FilesCab (G:\FilesCab).
4) Here you should see 6 cab files (Cab1.cab, Cab2.cab, ...)
5) Depending on which map(s) you need to restore, you will need to look in a certain Cabinet file. See below table.
Cabinet File Listings:
Cab1
a10.map
a30.map
a50.map
d40.map
Cab2
b30.map
b40.map
beavercreek.map
bloodgulch.map
boardingaction.map
Cab3
c10.map
c20.map
c40.map
carousel.map
chillout.map
d20.map
damnation.map
dangerground.map
deathisland.map
Cab4
bitmaps.map
Cab5
sounds.map
Cab6
Halo Directory Structure*(see note)
wizard.map
ui.map
timberland.map
sidewinder.map
ratrace.map
putput.map
prisoner.map
longest.map
infinity.map
icefields.map
hangemhigh.map
gephyrophobia.map
6) After you have figured out which Cab(s) contains the map which you need, open the cab using WinRAR or WinZIP (or your favorite cab-supporting compression program). Once its opened, navigate to the "maps" directory. Inside you will find the maps mentioned in the above table.
7) This should go without saying but...Copy and paste the map from the CAB archive into your Halo\Maps directory.

*NOTE: The Halo Directory structure, are the files and directories which get copied to your Halo directory upon installation. This is ONLY present in Cab6. As with the other cabs, in order to locate the original maps in this archive, you must first travel inside the Maps directory.
Hope this will keep duplicate posts on this subject to a bare minimum.