My computer keeps freezing

please can someone help me, my computer will hard freeze when it is running for a while, then when i restart it (manually) the freezing becomes more frequent. i just built the computer and put in a hard drive that is running xp pro i have 512 mb of ram and a p4 ht processor on a matsonic motherboard. the hard drive works fine when it is used on another computer everything is brand new. nobody could help me with this so any kind of help is greatly appreciated.