Windows XP Home crashes / Freezes

My son has a p3 1.4gHz pc with 256MB ram. He had a lot of problems with his computer crashing intermittently. We could not find it so I suggested he buy another hard drive and do a fresh install. He did this but he is still getting the crashes. What should he look for. He has formatted the new drive with NTFS. It is a 120Gb Maxtor drive. His graphic card is SIS AG315-64 ? motherboard not sure I need to look at it.