I have 18 gigs of hard drive, I would like to add 80 or 120 gigs more or replace the old hard drive. Is this a simple or hard job to take on? I have done some easy things such as increase RAM, replace CD R/W Drives, Putting is a new Modem card for high speed internet access. Please reply.

Thanks

Installing a new harddisk is nothing compared to works you have already done beleive me .

It's easy to install a new hard drive but not so easy to replace a hard drive.

To install a new hard drive just set the jumpers to SLAVE (should be instructions on the drive or the drive manual for that) and connect it to the other connector on the IDE cable that your current hard drive is on. Then connect the power.
When you boot/start your computer Windows should detect it and you'll have to format it before it'll be usable. If you cannot format it in Windows then use the FORMAT command in DOS. If your IDE cable only has one connector on it then go out and buy one with two, they cost almost nothing and most places will even give them to you for free.

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Replacing a hard drive is difficult unless you dont care if you lose all your data.

If you don't care about losing your data then...
Make sure the jumpers are set to MASTER.
Disconnect your old hard drive and connect the new one in the same way.
Start your computer, the new hard drive has no data on it and is not even readable yet so make sure you have a DOS boot disk (available through Windows 98, Windows Me, and Windows 2000 (I think)) or use a bootable CD like the Windows 2000 CD (I have one of these that is not bootable but they're supposed to be) or the Windows XP CD.
If you use the DOS boot disk, make sure you put FDISK on it manually or you won't have it. You need FDISK to create a new partition. If you use the Windows 2000 or WindowsXP CD then you don't need any DOS things because Windows will do it for you during the install process.
After you've used FDISK use the FORMAT command to format the drive and make it readable.
Proceed installing Windows.
After Windows is installed, install all your drivers and then all your programs.

If you want to keep your data you follow the same procedure except DO NOT install Windows. And before you power on your computer change the jumpers on the old drive to SLAVE and connect it as I told you how to connect another drive. Make sure you have a "ghost" program like Norton Ghost or something like that (Norton's the only one I've ever used).
A ghost program will copy ALL of one drive's contents and put it on another. It will also allow you to create "images" of your hard drive that can be backed up onto CD for incase your computer dies. Use the ghost program to create a DOS boot disk which will run the ghosting program or to create a DOS program disk (the program can be run from DOS).
Format the new drive using a DOS boot disk with FDISK and FORMAT.
Restart the computer with the ghosting program's boot disk (if it made one) or your other DOS boot disk.
If you started it with the ghosting program's boot disk then it should run the ghosting program for you. Use it to copy the contents of your old drive onto your new one. You'll have to figure this out for yourself because I don't have one running here to explain to you how and I don't know which program you will choose.
If you started with your DOS boot disk then insert the ghosting program's DOS disk and start the program. Then do the same thing as with if you started the program off its own boot disk.
Once all your data is copied, remove the old one and I guess you can sell it or something. Or you can reformat it and use it for extra storage (you can have up to 4 hard drives or other IDE devices (like CD-ROMs, DVD-ROMs, internal ZIP drives, etc) in your computer)(unless you have SCSI then the limit is so high that it doesn't even matter).

Hope I helped.
Good luck.
--Jon