Computer Frezzes

Hi, i need some help with my computer,

My PC Specs are:

AMD Sempron 2600 +
Motherboard Biostar M7VIG (Pro)
System Memory 1024 MB (PC3200 DDR SDRAM)
Video Adapter RADEON 9250 - Secondary (256 MB)
Disk Drive HDT722516DLA380 (160 GB, 7200 RPM, SATA-II)
Disk Drive WDC WD400BB-00DKA0 (37 GB, IDE)
Optical Drive HL-DT-ST DVDRAM GSA-4167B

The problem appers when i left the computer for a moment or when i left it downloading stuff from internet, the processor suddenly stops and it stays at 0% usage, the windows task manager appers when i press ctrl+alt+del but the processors does not react again and the start manu dont open again and finally i need to turn off the PC so it can starts again.

I dont know what seems to be the problem and i dont know what else can i do, i am thinking about changing my PC but it works well some times like rigth now and other days it just frezzes so i cant work.

Please help me.

:cry:confused