Dell has the on-board (motherboard) temps disabled.  You will need to use some separate monitoring unit, if you really need it. 

http://mbm.livewiredev.com/

Motherboard Monitor 5.3.6.0

 

Thanks but when installing the monitoring program it says Motherboard for Dimension 8300 is not supported

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Dimension 8300
P4 3.0Ghz
1GB DDR RAM PC3200
WD 80GB ATA-100
NVidia Geforce FX 5200 128MB DDR
SB Augity 2
CD-ROM
DVD-RWThanks