Its a standard 3.5 Eide PATA hard drive original spec is 80G.
Are you wanting to replace the internal drive for a larger one?
How big a drive do you want to fit?
Do you want to fit it as a second drive?
If first drive you may need to clone the original drive to replace windows and the restore partition.
You could always buy an external drive to plug into a USB port for more storage if require.