Access Hangs When Starting VBA

I've got a database which uses a lot of VBA. I've got the VBA Code in 1
mdb file with linked tables to the data in another mdb file so I can
keep code & data separate. All has been working fine until today when
in the code mdb if I try to open the VB Editor Access just hangs - CPU
usage goes to 100%. I've tried it on other mdb's and on earlier copies
of mdb in which the problem has occurred and it is fine. If I run the
code on the offending mdb all is ok - I just cannot get the editor to
open. Any ideas?

I'm on Access 2003 SP1, Win Xp Prof SP2 and am up to date on patches
etc.

Thanks