Am amazed it runs at all! XP takes most of the useful space on that HD . You should be able to fit almost any run of the mill HD, but 40Gb and below are almost non existent. SATA drives will not connect but you can confirm the type from Control Panel > System > Hardware.
You can download things like BelArc Advisor which is free and will give more detail of what is in the Computer. RAM for instance must be no more than the Motherboard will allow and must match the type it expects (Crucial will take you through RAM selection).
With luck the BIOS will support a larger HD and XP will find the mobo drivers for you, but you are going to have to install XP and all the applications again from their CDs.
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