I posted this in a thread a while ago and it just seemed too good to let go to waste so I'm posting it here in the tutorial forum:

Once it's powered off and unplugged, remove the hard drive from your other computer.
Set the jumper from it's current setting which is "Master" to "Slave.
On a Seagate/Maxtor or such drive it will look like this on the end:

| ::::::::::'::::::::: | :::: | ....
IDE Pins | Jumper Pins | Power connector
The Jumper Pins will look like this:

|::: (the first column will have the jumper), carefully remove the jumper and move it to the second column of pins so it's now like this :|::

If it's a Western Digital drive it's slightly different, the jumper pins will be like this
::::: and the jumper could be in one of two positions or there may not be one at all, in which case just remove the jumper from your CD/DVD drive and use it, remembering to put it back in when you're done. The way you need it to appear is this :::|: with the jumper on the second column from the power connector
Once that is done power off your present computer and unplug it, if you have an anti-static wristband please use it but if you don't no biggie just ground yourself by touching the case of the power supply, this will discharge any static electricity that may be built up in you, just in case...

Once you've opened the side panel of your computer you'll see the hard drive, connected to it is the IDE cable, it's about 1.25" wide and should be grey but might not be (I've seen blue ones as well). Don't touch the present HD or remove any cables from it, that's unnecessary. On the IDE cable should be another IDE connector (near the center of the cable or sometimes a few inches above the one on your present HD). Attach this connector to your other hard drive and find an available power plug and put it in the power slot (See image: http://pics.sangwara.info/images/A_8...0435_17689.jpg)

If you're just using it temporarily and plan on returning the HD to the original computer once finished then you can leave it just sitting in the case, if you plan on using it permanently in this computer than mount it to an available HD Mounting Rack in your case.
Again if it's temporary you don't need to put back on the side panel, just plug the PC in and Windows will say "Found New Hardware" and proceed to install the driver for that HD, then it should appear as the F drive in "My Computer" either named "Local Disk (F:)" or "Data Drive (F:)" Double click on it and navigate your way to your old files and copy them over!