I'll jump in here, in the off-chance that I can help without further info:
If it runs OK for a while, then suddenly locks up, it's often bad memory. There are other possibilities, such as an overheating processor.
For the processor, if you have no software to tell you what your fans are doing, it's a good idea to open the case and make sure that the processor fan (as well as any power supply fans) is turning at speed, and not covered with lint and that the heatsink beneath the fan isn't clogged.
For the memory, you need a memory checker, such as Memtest86+
, which will have to be downloaded and burned to a CD or floppy. For the CD, you need software that will burn an .iso file. Instructions at the site.