I am getting repeatedly this error when I startup my desktop (Windows XP SP2)

Error message is.......

The instruction at 0xfe9ed428 referenced memory at 0xfe9ed428. The memory
could not be read.
Click OK to terminate the program
Click CANCEL to debug the program

Once I click OK, I get the following message:

Windows Explorer has encountered a problem and needs to close. We are sorry
for the convenience.

When I click Don't Send button - the screen goes blank and again I get the
above message again.

I click Start and nothing happens. The only thing I can do is get the
Windows Task Manager (by Ctrl-Alt-Del) and do the shutdown.

Please Help - I cannot do anything!!!