Booting in XP: ntdll.dll

[skynet]

I am having alot of trouble booting in Windows XP. Its mainly a hit or miss type thing. I'll get a BSOD saying Hard error: ntdll.dll. I have no idea why this is happening but If I start in safe mode, then restard I can get in. Once I reset again I need to go back through safemode. Anyone have any Ideas?
-Thanks

[skynet]

I am also getting an unmountable boot volume error as well. Both of these error happen at random, someimte s I get in, sometimes I don't.

[Bires]

I have seen a similar error, during bootup on one of the computers in the school where I teach. I had no answer, and did have the time to investigate, but I am curious.