舊機換新既hard disk

當然可以換一個新硬碟入去啦！以你話係00年買的電腦，應該係插IDE(也是ATA)，跟住你就要睇下主機板晶片支援的最大容量(同樓上面那位人兄所講一樣)，你可以到出底板廠商的官網或者底板的說明書(應該有)睇睇，跟住就可以去買！

唔明再問！希望以上解答能夠幫到你！

your PC system must be kind of Intel Pentium III system (or compatible one).
then your motherboard can only support ATA (probably ATA100) harddisk.

It is more safe for you to buy a 80GB ATA100 harddisk which is around $300.

朋友：
　　請聯絡你底板／廠機的技術支援中心，核實它所能支
援硬碟最大的容量和照買就得，多數無問題，如有問題可
再到這裏。

雲遊四海，浪跡江湖。

It depends your motherboard supports which harddisk interface. You can refer to the manual of the motherboard, if it mentions supporting IDE or SATA. Or you can check from internet.