How can I fix My Dell GX 260 ?

A month ago I bought a used Dell GX 260, everything was orderly. But suddenly the power butten stopped to switch the computer on. To turn the pc on I 've to always take out the CMOS batrrey and then put it back. Even then the computer starts by it self, the power swicth is no more working. I formated it using Win .XP. SP2 (2096), no change, even worse the mou se curser is no more there. When I try to Add the mouse in the control panel , it says the device is working properly . Please help I need my PC back soon. Tanx.