Knowing the model number of the computer would help with the specifics, but:
1.) Make sure you have a bay for the CDROM drive and a slot for the hard drive...without space to put them you can't do anything. If there's no room you'd have to buy externals or buy cases to make the internals externals.
2.) Make sure that your PSU (power supply unit) can handle the extra drives. If not that's something you'd have to upgrade first.
3.) Pick out/purchase a CDROM (are you sure you want a CD-ROM and now a CD burner or DVD drive?) and hard drive. Make sure it's not a SATA drive, as I doubt your computer would support it.
4.) Install the drives by following the instructions that come with the drives...crack open your computer's case, slide the drives in, screw them in, and attach the power and data cables.
5.) Install any software/drivers, as necessary.
6.) Format the new hard drive so that you can use it. (Windows won't 'see' the drive until it's been formatted.)
Of course a lot of that will depend on your computer's design (known from the model number), followed by the drives you choose, but that's a good summary.
Hope this helps,
ok i am using windows ME and have a pentium 4 2.6 all i want to do is install a second hard drive and a second cdrom!!! what do i have to do?