It's like asking will the new car door I'm gonna buy fit my old car ? We don't know what door you're buying or the car you have now to put it on.
Let's start over...tell us what you're trying to do.
Replace an existing hard drive..?
Add another hard drive ?
How large a hard disk in GBs are you trying to add/replace ?
i have old computer p-3 750 mhz with 20g hd.
how can i know that my new hd that i will buy, fit to my old computer?