ArcHydro woes

Hi All

I am trying to use ArcHydro with ArcView 9.1. Up until this morning it
was working perfectly. Now I keep getting the following when trying to
recondition the DEM.

'-2147217366, Error during conversion', 'Error occurred while DEM
Reconditioning (AGREE)!'

Does this make sense to anyone. I think it is something to do with
Spatial Analyst but that works fine outside ArcHydro. I have done a
complete reinstall of Arc and ArcHydro and it has made no difference.
No other software has been installed on this machine (unless the
company network has added some XP update as it seems to do every so
often). It isn't a problem with the data as it works fine on other
machines (that are not networked).

Any help appreciated

Al