How to fix system locking up issue?

Hi,
I have an Intel Pentium 4 PC and I am running window XP home edition on it. I am being teased by an issue which is my pc use to locks up exactly after the welcome screen. Mean, whenever I start my computer it locks up right after starting till welcome screen. I have tried all the tricks I know, to get rid of this problem but I am unable to fix it so far. While tying to fix this issue, I thought that this might be due to totally filled RAM memory with apps loading on system boot-up. And I freed-up the RAM memory but no success for me.
I also have tried covering this issue by starting my computer in safe mode but it didn’t work either. I am suspecting for a bad memory chip but hope that’s not the problem with my PC. This problem is disturbing me a lot and I want the solution for it as early as possible. So, I need to know How to fix system locking up issue? If anybody has an idea about this then kindly help me to get out of this. Am waiting anxiously, for some useful suggestions.
