Ive just installed a SATA II drive on my system, after the PATA gave me problems..
now whenever i run demanding programs such as Photoshop, Audio processing programs, DC++ server or demanding games( BF 2142, UT 2004) the sytem just freezes .. and i have to reset the system
this has never happened before , only when i ve installed the new SATA drive..
specs are as follows:

Intel Prescott P4 3GHz Ht CPU
Intel 915GAV MB
Toshiba 200Gb SATA II drive
Toshiba PATA 80Gb drive
1Gb RAM in dual channel
Radeon X300 PCI E screen card ( oveclocked when playng games )
Windows XP pro SP2

You didn't specify the PSU power. Might just be the lack of it.

Try running things without 80 G drive to reduce the load on PSU.

300W PSU

ran it without the 80Gb..

found out last night that the SATA was faulty..
it bummed out.. and made a scratchy sound..

phoned the retailer this morning to get it replaced..

i might still replace the PSU.. for cooling purposes...

300W?? Are you sure? Haven't seen 300W psu since 386-es. Man, replace it with 500W MINIMUM. No wonder you have shutdowns.

P.S. Don't let that retailer find out abut your PSU. Might just void the warranty.