problem with friends pc

[etones]

my friend is having a really weird problem with his pc, when he turns it on it shows the company logo then windows xp is ment to load but i doesnt nothing happens but if you press a few keys on the keyboard it will start to load, then after that it works fine. sometimes pressing the keyboard doesnt do anything nothing happens and sometimes when it gets to the login screen it says windows is shuting down. he doesnt know much about computers so thats why he asked me. pc specs below -

Compaq Presario SR1500
windows xp home
AMD Athlon™ 64 processor 3200+
512mb ram