Ubuntu file system going into "lockdown" mode

[edwardp-mh2Nk+tgbQM at public.gmane.org]

I have Ubuntu 10.04.1 LTS installed on a 32-bit AMD Athlon system, 
memory is maxed out at 512Mb.  On occasion, the system will stop 
running, when I go into the file manager, all of the icons have locks on 
them, meaning it has gone into read-only (lockdown) mode.

Rebooting the system will take it into a file system check and will 
prompt to fix the errors (if there were any).

I have already run the memory test on this and it has reported no errors 
with the memory.  Could there be something else hardware-related that 
causes this to occur??  It is an older system though, but with the LXDE 
desktop, it runs quite fast on the Athlon CPU.

Thanks in advance...