XenApp Server Registry Error

Hi, has anyone got this error before:

Event ID 333
Source: Application pop up
Type: Error

An I/O operation initiated by the Registry failed unrecoverably. The
Registry could not read in, or write out, or flush, one of the files that
contain the system’s image of the Registry.

This is a Win2003Std32 server with Citrix XenApp 4.5.

Any ideas as to why???