|It is connected and installed but not formatted.|
Please go to device manager and go to the Hard Drive section and there you will be able to see the HDD. You have to format it from there to be able to see it in My Computer..
And why don't you upgrade to Windows 7. It is much faster then Win XP on an old machine aswell. I installed Windows 7 on a P4 1.6Ghz with 1GB RAM and 40GB HDD and the PC became much faster in overall performance including opening programs and booting up.
Please see my website you will be able to see some info regarding formatting a hard drive.
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