There are two types of CD-ROM drives we can mod:
24x to 40x
40x or higher
Drives slower than 24x are too old to be modded.
24x to 40x CD ROMs

Laser Head Adjustments:
The
track pitch of CDs is 1.6 microns, and the track pitch of DVDs is only
0.8 microns. The minimum length of track pits of CDs is 0.843mm, and
the minimum length of track pits of DVDs is 0.293mm. That's why DVD
discs can store much more data in the size of a CD.
Pinpointing this
situation, we need to adjust the laser head so it can read discs with
smaller track pitchs and shorter track pits. Open up the CD-ROM drive,
do you see a lens on the rail? That's the laser head. On the side of
the laser head, there's a screw you can adjust.This is the key of this
mod.This screw can adjust the size of the laser beam that lands on the
disc. Referring to the above, after adjusting, the laser beam should be
less than 0.293mm, to suit the needs of DVD discs. So turn that screw
2-3 times. I got this number through trial and error. Mark with a
pencil, in case you forget how many times you turned that screw.

Speed adjustments:
DVD-ROM
drives can play DVD movies smoothly at 4x speed. If we're modding a 32x
CD-ROM drive, the rotation speed is obviously too high, increasing
heat, and shortening the life of the drive.
So we need to decrease the speed of it.
Most
people know that power supplies can provide 5V (red wire) and 12V
(yellow wire) electricity output for Molex connectors. Find the Molex
connector you'll plug into the modded CD-ROM drive, cut the yellow wire
or insulate it with tape, so only 5V of electricity is transferred to
the CD-ROM drive. The speed of the drive is now 32 * 5/17 = 9.41x and
can now fulfill our requirements.

40x or higher CD ROMs
When
40x CD-ROM drives are released, most manufacturers are already
producing DVD-ROM drives. To lower cost, they use the same core as
DVD-ROM drives with DVD functions disabled. What we need to do here is
to re-enable the DVD function.
Open up the CD-ROM, behind the
circuit board, look for a jumper that says DVD JUMP. Find a jumper to
connect this jumper. Thin metal wire also works fine.
OK, so the DVD
function is unlocked, but we still need to decrease the speed. Use the
instructions above on how to decrase the speed of the drive.

Notes:
Modded
drives cannot be detected in POST, that means you cannot use it in DOS.
But once you get into Windows, the drive works fine.
Modded drives have no region code problems.
Modded drives MIGHT NOT read DVD9 discs, that's why I said you can watch most retail DVDs in the beginning.