First make sure the disk is jumpered as a master, since this is an older system and the cable is probably also the older 40 pin type. The drive may have a picture on it showing how to set the jumper for master. If it is an 80 pin cable, then set the jumper to cable select (CS).
If it does not automatically recognize your new drive, you will have to get in to the bios (typically by hitting the delete key or one of the function keys )to see if it will detect it. That is usually the first screen and will tell you the drive that are connected.
Depending on the age of your motherboard there could be issues with the hard drive (size) and with the XP O/S that is installed on it. The O/S will think its still part of the other hardware system.
Since you have an O/S installed on that drive, if it does manage to boot (no guarantee it will, especially XP), it is going to want drivers for whatever devices you have on your system and for the motherboard. This may require some research and downloading off the Internet if you do not have them. Things like the video card, sound card, chipset, modem or network may all be different and you may have to provide the drivers.
If you get the system to recognize the drive, it may be easier to do a new install. Transfering an O/S between different hardware configurations can often be very problematic.
I want to install a new drive (used but new to this unit) on a 4 year old Acer unit and have not been able to do so. I removed the old 3gb drive and installed my 10gb but on start up the Acer name screen comes on and thats as far as it goes. I cannot get it to the actual point of accepting the drive or starting it. The os on the drive is WinXP Home. Ive heard I need to refigure BIOS but lack the knowledge to do so. I can enter the BIOS but dont know the method for bypassing or removing the Acer components embedded therein. Any help would be appreciated greatly. Arthur