Excel2007
One of my sheets of reasonable size (last cell used IK1389) all of a sudden
started to display and scroll very slowly (+30s to page fwd).
I found that there are 100s (maybe 1000s - too many to count) of conditional
formats which vaguely resembled one format that I may have entered some time
ago. - Anyway, I don't want them anymore.
I suspect that they could be responsible for the sluggish behaviour.
Now I also found that when i select the whole sheet, the "Manage Rules"
window shows the majority of these rules with an empty "Applies to".

For curiosity I tried to delete one of these rules but Excel appeared to be
frozen - I cancelled it via the task manager.

Questions:
1) How can I clear all conditional formatting from a sheet without having to
go rule by rule and without interfering with (ie keeping) all other
formatting on the sheet?
2) Where could these "Applies to = blank" rules come from?

Thanks for your help,
Michael