bash: fork: Permission denied -- how can I fix this?

I have recently installed Cygwin version 2.510.2.2 onto a Windows XP system and when I launch it the first thing I see is this:

      5 [main] bash 3024 fork: child -1 - CreateProcessA failed, errno 13
bash: fork: Permission denied

I get the same message when I try running the ls command:

bash-3.2$ ls
9404505 [main] bash 1544 fork: child -1 - CreateProcessA failed, errno 13
bash: fork: Permission denied

Can someone suggest what's going wrong, and how I can fix it?

Thanks in advance for any help.

--James