I strongly recomend to buy a new one.You can stay your hard disk,mouse,keyboard,DVD ROM,box...etc.
If you have budget about NT$5000,I advise you
cpu:AMD Athelon64x2 4400
motherboard:ASUS M2N-MX SE PLUS
RAM:DDR2 800 1GB
If your budget is more than NT$5000,you can consider to adjust.